Liquid crystal display apparatus

ABSTRACT

A liquid crystal display apparatus includes a liquid crystal display panel including a liquid crystal cell between first second polarizing plates; and a surface light source device. The surface light source device includes a light source unit, a light guide plate for outputting first directivity light having directivity of maximum intensity in a first direction, which forms a predetermined angle from a normal direction of the light output surface in a plane approximately parallel to a light guiding direction of light, and having a high ratio of a polarized light component oscillating in the plane and a prism sheet being configured to convert the first directivity light into second directivity light directed in a second direction within a predetermined angle from the normal direction of the light output surface of the light guide plate while substantially maintaining a polarization state of the first directivity light.

TECHNICAL FIELD

The present invention relates to a liquid crystal display apparatus.

BACKGROUND ART

In recent years, as a display, a liquid crystal display apparatus usinga surface light source device has been remarkably widespread. As theliquid crystal display apparatus using the surface light source device,for example, a liquid crystal display apparatus including an edgelight-type surface light source device is known. In such a liquidcrystal display apparatus, light emitted from a light source enters alight guide plate, and propagates through an inside of the light guideplate while repeating a total reflection on a light output surface(liquid crystal cell-side surface) of the light guide plate and a backsurface thereof. A part of the light that propagates through the insideof the light guide plate allows a traveling direction thereof to bechanged by a light scattering body or the like, which is provided on theback surface of the light guide plate or the like, and is output fromthe light output surface to an outside of the light guide plate. Suchlight output from the light output surface of the light guide plate isdiffused and condensed by various optical sheets such as a diffusionsheet, a prism sheet, a brightness enhancement film, or the like, andthereafter, the light enters a liquid crystal panel in which polarizingplates are arranged on both sides of a liquid crystal cell. Liquidcrystal molecules of a liquid crystal layer of the liquid crystal cellare driven for each of pixels to control transmission and absorption ofthe incident light. As a result, an image is displayed.

As described above, the liquid crystal panel includes the polarizingplates on both sides (front and back) thereof, accordingly,approximately a half of the light that enters the liquid crystal panelis absorbed by the polarizing plate on the incident side, andutilization efficiency of the light in the liquid crystal panel isessentially low. Accordingly, when a larger amount of light is attemptedto be entered into the polarizing plate in order to obtain desiredbrightness, there are such various problems that heat from the lightsource adversely affects the liquid crystal and the like, resulting indifficulty viewing display, in addition to a problem in that a powerconsumption of the light source is increased.

Various proposals have been made in order to improve such lightutilization efficiency in the liquid crystal display apparatus. As oneof the proposals, there is a proposal, in which non-polarized light fromthe light source is separated by using a polarized light separating bodythat separates the non-polarized light into two pieces of linearlypolarized light, which are in a perpendicular relationship to eachother, by transmission and reflection, one piece of the polarized lightthus separated is transmitted and directly used, and in addition, theother piece of the reflected polarized light is also reused. That is,this is a technology in which, among polarized light componentsseparated by the polarized light separating body, one of the polarizedlight components, which is transmitted, allows a polarization directionof the transmission light and a transmission axis direction of a lowerpolarizing plate (incident-side polarizing plate) to coincide with eachother, and is caused to enter the liquid crystal cell, and otherpolarized light component is returned to the light source side, linearlypolarized state of the returned light is eliminated therefrom bybirefringence, reflection, diffraction or diffusion, or the like, andthe light is guided to the polarized light separating body again and isreused so that the light utilization efficiency is enhanced. Forexample, in Patent Literature 1, a backlight is described, in which sucha light control sheet that allows output light to be substantiallyvertical to a surface of the planar light guide plate is provided on alight output surface side of the planar light guide plate, and polarizedlight separating means is further arranged thereon.

However, in the backlight described in Patent Literature 1, a structureof the polarized light separating body is complicated, and inparticular, it is difficult to form a polarized light separating layeron an inclined surface portion of a columnar prism array that istriangular in cross section, with the result that mass productivity ofthe backlight is insufficient. In recent years, there has also beendeveloped a surface light source device constructed so that the lightoutput from the light guide plate has a predetermined polarization stateor the like. However, output light that is the polarized light outputfrom the light guide plate is not utilized sufficiently, effectively,and sufficient brightness is not obtained.

Moreover, in Patent Literature 2, it is described that light polarizedin a predetermined direction is output from a surface light source, anda base having a birefringence ratio, such as a biaxially stretched film,is used as a base of a prism sheet, and thus the polarization directionof the polarized light that enters the liquid crystal panel iscontrolled to reduce the light absorbed to the polarizing plate andenhance the utilization efficiency of the light. However, also in theliquid crystal display apparatus of Patent Literature 2, the utilizationefficiency of the light is still insufficient.

CITATION LIST Patent Literature

[PTL 1] JP 06-265892 A

[PTL 2] JP 4673463 B2

SUMMARY OF INVENTION Technical Problems

The present invention has been made in order to solve theabove-mentioned problems and it is an object of the present invention toprovide a liquid crystal display apparatus, which has high utilizationefficiency of light, and is capable of displaying a bright image.

Solution to Problems

A liquid crystal display apparatus according to an embodiment of thepresent invention includes: a liquid crystal display panel including aliquid crystal cell between a first polarizing plate provided on aviewer side and a second polarizing plate provided on a back surfaceside; and a surface light source device for illuminating the liquidcrystal display panel from the back surface side. The surface lightsource device includes: alight source unit; a light guide plate forcausing light from the light source unit to enter from a light incidentsurface opposed to the light source unit, and for outputting firstdirectivity light from a light output surface opposed to the liquidcrystal display panel, the first directivity light being polarized lighthaving directivity of maximum intensity in a first direction, whichforms a predetermined angle from a normal direction of the light outputsurface in a plane approximately parallel to a light guiding directionof light, and having a high ratio of a polarized light componentoscillating in the plane; and a prism sheet arranged on a liquid crystaldisplay panel side with respect to the light guide plate, the prismsheet including a prism portion in which a plurality of columnar unitprisms protruding on a light guide plate side are arrayed, the prismsheet being configured to convert the first directivity light intosecond directivity light directed in a second direction within apredetermined angle from the normal direction of the light outputsurface of the light guide plate while substantially maintaining apolarization state of the first directivity light. A ridge linedirection of the columnar unit prisms is approximately parallel to thelight incident surface of the light guide plate, a transmission axis ofthe second polarizing plate is approximately parallel to a polarizationdirection of the second directivity light and the light guidingdirection of the light in the light guide plate, and the transmissionaxis of the second polarizing plate is approximately perpendicular to atransmission axis of the first polarizing plate.

In one embodiment of the present invention, the prism sheet includes abase portion on the liquid crystal display panel side with respect tothe prism portion, the base portion supporting the prism portion, andsubstantially having optical isotropy.

In one embodiment of the present invention, the first directivity lightcontains, by 52% or more, the polarized light component oscillating inthe plane approximately parallel to the light guiding direction of thelight.

In one embodiment of the present invention, the light guide platecontains a light scattering material. The light guide plate includes aplurality of columnar back surface-side unit optical elements protrudingon a back surface side of the light guide plate, the plurality ofcolumnar back surface-side unit optical elements being arrayed from thelight incident surface side of the light guide plate to a side surfaceon an opposite side, and an array direction of the columnar unit prismsis approximately parallel to an array direction of the plurality ofcolumnar back surface-side unit optical elements.

In one embodiment of the present invention, the liquid crystal displayapparatus further includes a polarized light selective reflection sheetbetween the surface light source device and the first polarizing plate.

Advantageous Effects of Invention

According to one embodiment of the present invention, the liquid crystaldisplay apparatus, which has the high utilization efficiency of thelight, and is capable of displaying the bright image, can be provided.As a result, a power consumption of the light source unit can bereduced, for example, by reduction of the number of light sources and/orreduction of outputs of the light sources. Moreover, the number ofmembers for enhancing the utilization efficiency of the light can bereduced, and accordingly, great advantages are brought from anyviewpoint of cost, manufacturing efficiency, and thinning. Inparticular, favorable display characteristics can be maintained evenwhen the polarized light selective reflection sheet that is extremelyexpensive is omitted. The thinning is realized so that a designselection range can be expanded to a large extent, and a liquid crystaldisplay apparatus that is commercially valuable can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic perspective view illustrating a liquid crystaldisplay apparatus according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view of a liquid crystal cell foruse in the liquid crystal display apparatus of FIG. 1.

FIGS. 3( a) and 3(b) are schematic cross-sectional views illustratingalignment states of liquid crystal molecules in a VA mode.

FIGS. 4( a) and 4(b) are schematic cross-sectional views illustratingconfigurations of a surface light source device in the liquid crystaldisplay apparatus of FIG. 1.

FIGS. 5( a) and 5(b) are schematic views illustrating shapes of a lightoutput-side unit optical element and a back surface-side unit opticalelement of a light guide plate of the surface light source device ofFIGS. 4( a) and 4(b).

FIGS. 6( a) and 6(b) are views illustrating states of light output fromthe light guide plate and a prism sheet.

FIGS. 7( a) to 7(c) are graphs showing various relationships among anincident angle, a P component, and an S component.

FIG. 8 is a schematic view illustrating a unit prism of the prism sheetof the surface light source device of FIGS. 4( a) and 4(b).

FIG. 9 is a schematic view illustrating a unit prism according toanother embodiment.

FIGS. 10( a) and 10(b) are graphs showing an intensity distribution ofbrightness of first directivity light L1 output from the light guideplate and an intensity distribution of brightness of second directivitylight L2 output from the prism sheet in one embodiment of the presentinvention.

FIGS. 11( a) to 11(d) are views illustrating relationships amongpolarization directions of the output light from the light guide plateand the prism sheet, and a transmission axis of a first polarizing plateand a transmission axis of a second polarizing plate in one embodimentof the present invention.

FIG. 12 is a schematic view illustrating a shape of a modified form ofthe unit prism.

FIG. 13 is a schematic view illustrating a shape of a light output-sideunit optical element of a light guide plate used in examples of thepresent invention.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is described below with referenceto the drawings and the like.

Note that, the respective drawings to be referred to below, whichinclude FIG. 1, are schematic views, and sizes and shapes of therespective portions are exaggerated as appropriate in order tofacilitate the understanding.

Moreover, words such as“plate”, “sheet”, and“film” are used. However, ingeneral usage, the “plate”, the “sheet”, and the “film” are used in thestated order, which is a descending order of thickness, and also in thisspecification, are used in accordance with this order. However, suchusage has no technical meaning, and accordingly, the word is unified as“sheet” for use in the description of the claims. Thus, it is definedthat the words, the“sheet”, the“plate”, and the “film”, can be replacedas appropriate. For example, a prism sheet may be expressed as a prismfilm or a prism plate.

Further, dimensional numeric values, material names and the like of therespective members, which are described in this specification, aremerely examples, but without being limited thereto, may be selected asappropriate for use.

FIG. 1 is a view illustrating a liquid crystal display apparatus 1according to an embodiment of the present invention. The liquid crystaldisplay apparatus 1 of this embodiment includes a surface light sourcedevice 20 and a liquid crystal display panel 15 configured to beilluminated from a back surface thereof by the surface light sourcedevice 20. Note that, though a description or the like is omitted, inaddition to those components, the liquid crystal display apparatus 1includes usual devices, such as wires, circuits, and members, which arerequired in order to operate as a liquid crystal display apparatus.

Note that, in the drawings and the following description, in order tofacilitate the understanding, in a usage state of the liquid crystaldisplay apparatus 1, a direction perpendicular to a light guidingdirection of light in a light guide plate is defined as an X direction,the light guiding direction of the light in the light guide plate isdefined as a Y direction, and a normal direction of a viewing screen isdefined as a Z direction. A viewer visually recognizes display of ascreen of the liquid crystal display panel 15 from a Z2 side serving asa viewer side toward a Z1 side serving as a back surface side. Moreover,in a thickness direction (Z direction) of a prism sheet 30 and theliquid crystal display panel 15, the Z1 side is an incident side of thelight, and the Z2 side is an output side of the light.

The liquid crystal display panel 15 is a transmission-type image displayunit, and includes a first polarizing plate 13 arranged on the viewerside (output side, Z2 side), a second polarizing plate 14 arranged onthe surface light source device 20 side (Z1 side), and a liquid crystalcell 12 arranged between the first polarizing plate 13 and the secondpolarizing plate 14. Each of the polarizing plates has functions todecompose light, which enters the polarizing plate, into two polarizedlight components perpendicular to each other, to transmit therethroughthe polarized light component in one direction (direction parallel to atransmission axis), and to absorb the polarized light component in adirection (direction parallel to an absorption axis) perpendicular tothe one direction. In this embodiment, the transmission axis of thesecond polarizing plate 14 and the transmission axis of the firstpolarizing plate 13 are substantially perpendicular to each other whenviewed from a front direction of the liquid crystal display panel 15(front direction of the viewing screen of the liquid crystal displayapparatus 1). In this embodiment, for example, the transmission axis ofthe first polarizing plate 13 is in the X direction, and thetransmission axis of the second polarizing plate 14 is in the Ydirection. As described above, the X direction is the directionperpendicular to the light guiding direction of the light in the lightguide plate, and is a right-and-left direction of the screen in theexample of the figure. As described above, the Y direction is the lightguiding direction of the light in the light guide plate, and is anup-and-down direction of the screen in the example of the figure. Thetransmission axis of the second polarizing plate 14 is substantiallyparallel to a light guiding direction of the light in a light guideplate 21 to be described later. Note that, in this specification, theexpressions “substantially perpendicular” and “approximatelyperpendicular” include a case where an angle formed by two directions is90°±10°, preferably 90°±7°, more preferably 90°±5°. The expressions“substantially parallel” and “approximately parallel” include a casewhere an angle formed by two directions is 0°±10°, preferably 0°+7°,more preferably 0°±5°. Moreover, in this specification, such a simpleexpression “perpendicular” or “parallel” can include a substantiallyperpendicular state or a substantially parallel state.

Referring to FIG. 2, the liquid crystal cell 12 of this embodimentincludes a pair of substrates 121 and 121′ and a liquid crystal layer122 as a display medium sandwiched between the substrates. In a generalconfiguration, on the substrate 121 as one in the pair, a color filterand a black matrix are provided, and on the substrate 121′ as the otherin pair, there are provided switching elements for controllingelectro-optical property of the liquid crystal, scanning lines forgiving gate signals to the switching elements and signal lines forgiving source signals thereto, and pixel electrodes and counterelectrodes. An interval (cell gap) between the above-mentionedsubstrates 121 and 121′ can be controlled by spacers and the like. Onsides of the above-mentioned substrates 121 and 121′, which are broughtinto contact with the liquid crystal layer 122, for example, alignmentfilms made of polyimide or the like can be provided.

In one embodiment, the liquid crystal layer 122 includes liquid crystalmolecules aligned in a homogeneous alignment in a state where anelectric field is not present. The liquid crystal layer (liquid crystalcell as a result) as described above typically exhibits athree-dimensional refractive index of nx>ny=nz in a case whererefractive indices of the liquid crystal layer in a slow axis direction,a fast axis direction, and a thickness direction are nx, ny, and nz,respectively. Note that, in this specification, ny=nz includes not onlya case where ny and nz are completely the same, but also a case where nyand nz are substantially the same.

As a typical example of a drive mode using the liquid crystal layer thatexhibits the three-dimensional refractive index as described above, thein-plane switching (IPS) mode, the fringe field switching (FFS) mode,and the like are given. In the above-mentioned IPS mode, by using theelectrically controlled birefringence (ECB) effect, the liquid crystalmolecules aligned in the homogeneous alignment in the state where anelectric field is not present are allowed to respond, for example, to anelectric field (also referred to as a horizontal electric field), whichis generated by the counter electrode and pixel electrode, each beingformed of metal, and is parallel to the substrates. More specifically,for example, as described in “Monthly Display, July” pp. 83 to 88(1997), published by Techno Times Co., Ltd. and “Ekisho vol. 2, No.4″pp. 303 to 316 (1998), published by The Japanese Liquid CrystalSociety Publishing”, when an alignment direction of the liquid crystalcell at the time when no electric field is applied thereto and anabsorption axis of a polarizer on one side are allowed to coincide witheach other, and the upper and lower polarizing plates are arrangedperpendicularly to each other, the normally black mode providescompletely black display in the state where no electric field ispresent. When the electric field is present, the liquid crystalmolecules perform a rotation operation while remaining parallel to thesubstrates so that a transmittance corresponding to a rotation angle canbe obtained. Note that, the above-mentioned IPS mode includes the superin-plane switching (S-IPS) mode and the advanced super in-planeswitching (AS-IPS) mode, each of which employs a V-shaped electrode, azigzag electrode, or the like.

In the above-mentioned FFS mode, by using the electrically controlledbirefringence effect, the liquid crystal molecules aligned in thehomogeneous alignment in the state where no electric field is presentare allowed to respond, for example, to an electric field (also referredto as a horizontal electric field), which is generated by the counterelectrode and pixel electrode, each being formed of a transparentconductor, and is parallel to the substrates. Note that, the horizontalelectric field in the FFS mode is also referred to as a fringe electricfield. This fringe electric field can be generated by setting aninterval between the counter electrode and the pixel electrode, each ofwhich is formed of the transparent conductor, narrower than the cellgap. More specifically, for example, as described in “SID (Society forInformation Display) 2001 Digest, pp. 484 to 487” and JP 2002-031812 A,when an alignment direction of the liquid crystal cell at the time whenno electric field is applied thereto and an absorption axis of apolarizer on one side are allowed to coincide with each other, and theupper and lower polarizing plates are arranged perpendicularly to eachother, the normally black mode provides completely black display in thestate where no electric field is present. When the electric field ispresent, the liquid crystal molecules perform a rotation operation whileremaining parallel to the substrates so that a transmittancecorresponding to a rotation angle can be obtained. Note that, theabove-mentioned FFS mode includes the advanced fringe field switching(A-FFS) mode and the ultra fringe field switching (U-FFS) mode, each ofwhich employs a V-shaped electrode, a zigzag electrode, or the like.

In the drive mode (for example, the IPS mode, the FFS mode) using theliquid crystal molecules aligned in the homogeneous alignment in thestate where no electric field is present, there is no oblique gray-scaleinversion, and an oblique viewing angle thereof is wide, andaccordingly, there is an advantage in that visibility in an obliquedirection is excellent even when the surface light source directed inthe front direction, which is used in the present invention, is used.

In another embodiment, the liquid crystal layer 122 includes liquidcrystal molecules aligned in a homeotropic alignment in the state whereno electric field is present. As a drive mode using the liquid crystalmolecules aligned in the homeotropic alignment in the state where noelectric field is present, for example, the vertical alignment (VA) modeis given. The VA mode includes the multi-domain VA (MVA) mode.

FIGS. 3( a) and 3(b) are schematic cross-sectional views illustratingalignment states of the liquid crystal molecules in the VA mode. Asillustrated in FIG. 3( a), the liquid crystal molecules in the VA modeare aligned, at the time when no voltage is applied thereto,approximately vertically (normal direction) on the substrates 121 and121′. Here, the term “approximately vertical” also includes a case wherean alignment vector of the liquid crystal molecules is inclined withrespect to the normal direction, that is, a case where the liquidcrystal molecules have a tilt angle. The tilt angle (angle from thenormal line) is preferably 10° or less, more preferably 5° or less,particularly preferably 1° or less. The liquid crystal molecules havethe tilt angle in such a range so that the liquid crystal displayapparatus can be excellent in contrast. Moreover, moving picture displaycharacteristics can be enhanced. The approximately vertical alignment asdescribed above can be realized, for example, by arranging nematicliquid crystal, which has negative dielectric anisotropy, betweensubstrates on which vertical alignment films are formed. When lightenters from a surface of the one-side substrate in such a state, lightof linearly polarized light, which passes through the second polarizingplate 14 and enters the liquid crystal layer 122, travels along adirection of a major axis of the liquid crystal molecules alignedapproximately vertically. The birefringence is not generatedsubstantially in a major axis direction of the liquid crystal molecules,and accordingly, the incident light travels without changing apolarization direction thereof, and is absorbed by the first polarizingplate 13 having a transmission axis perpendicular to the secondpolarizing plate. In this manner, display of a dark state is obtained atthe time when no voltage is applied (normally black mode). When avoltage is applied between the electrodes, the major axis of the liquidcrystal molecules is aligned parallel to the substrate surfaces. Theliquid crystal molecules in this state exhibit the birefringence to thelight of the linearly polarized light, which passes through the secondpolarizing plate 14 and enters the liquid crystal layer, and thepolarization state of the incident light is changed in response to aninclination of the liquid crystal molecules. The light that passesthrough the liquid crystal layer 122 at a time when a predeterminedmaximum voltage is applied becomes, for example, linearly polarizedlight in which a polarization direction is rotated by 90°, andaccordingly, the light transmits through the first polarizing plate 13,and display of a bright state is obtained. When the state where novoltage is applied is set again, the display can be returned to thedisplay of the dark state by alignment regulating force. Moreover, theinclination of the liquid crystal molecules is controlled by changingthe applied voltage, and transmission intensity of the light from thefirst polarizing plate 13 is changed so that gray-scale display becomespossible. In the case of the VA mode, a transmittance in middle tones inthe oblique direction is higher than a transmittance in middle tones inthe front direction, and accordingly, there are advantages in that themiddle tones viewed obliquely are bright even when the surface lightsource directed in the front direction, which is used in the presentinvention, is used, resulting in a small amount of blocked-up shadows.

FIGS. 4( a) and 4(b) are views illustrating a configuration of thesurface light source device 20 of this embodiment. FIG. 4( a) is across-sectional view of the surface light source device 20 taken alongthe line indicated by the arrows A1-A2 in FIG. 1, and FIG. 4 (b) is across-sectional view of the surface light source device 20 taken alongthe line indicated by the arrows B1-B2 in FIG. 1. As illustrated in FIG.1, the surface light source device 20 is a lighting device, which isarranged on the back surface side (Z1 side) with respect to the liquidcrystal display panel 15, for illuminating the liquid crystaldisplaypanel 15 from the back surface side. As illustrated in FIG. 1 andFIGS. 4( a) and 4(b), the surface light source device 20 is an edgelight-type surface light source device (backlight), which includes thelight guide plate 21, a light source unit 10, the prism sheet 30, and areflection sheet 11. The surface light source device 20 may be what iscalled a single lamp-type surface light source device, in which thelight source unit 10 is arranged along one side surface (21 a or 21 b ofFIG. 1) of the light guide plate 21, or may be what is called a duallamp-type surface light source device, in which the light source unit 10is arranged along two opposed side surfaces (21 a and 21 b of FIG. 1) ofthe light guide plate 21. As illustrated in FIG. 4( a), in thisembodiment, the dual lamp-type surface light source device isillustrated. Note that, as illustrated in FIG. 1, the surface lightsource device 20 may adopt a form in which a polarized light selectivereflection sheet 16 is provided between the prism sheet 30 and theliquid crystal display panel 15. The polarized light selectivereflection sheet 16 transmits therethrough polarized light in a specificpolarization state (polarization direction), and reflects light in otherpolarization states. The polarized light selective reflection sheet 16is arranged so as to transmit therethrough light in a polarizationdirection parallel to a polarizing axis of the second polarizing plate14, and can thereby reuse the light absorbed to the second polarizingplate 14, can further increase the utilization efficiency, and moreover,can enhance the brightness.

The light guide plate 21 is a member configured to guide the light,which enters from the light source unit 10, to an end side opposed tothe light source unit 10 side while receiving a reflection action or thelike in the light guide plate 21, and gradually output the light from alight output surface 21 d (surface on the prism sheet 30 side) in thelight guide process. The light guide plate 21 includes a base portion22, a light output-side unit optical element portion 23, and a backsurface-side unit optical element portion 25. The base portion 22 is asheet-like member, and has light transmissivity.

As illustrated in FIG. 1 and FIGS. 4( a) and 4(b), the light output-sideunit optical element portion 23 is formed on a surface of the baseportion 22 on the prism sheet 30 side (Z2 side). On the lightoutput-side unit optical element portion 23, a plurality of lightoutput-side unit optical elements 24 are arrayed in parallel to oneanother. The light output-side unit optical elements 24 are columnar,maintain a cross-sectional shape that appears on a cross sectionillustrated in FIG. 4( b), and define the direction (Y direction) ofguiding light as a longitudinal direction. The plurality of lightoutput-side unit optical elements 24 are arrayed in parallel to oneanother in a direction (X direction) perpendicular to this longitudinaldirection.

FIGS. 5( a) and 5(b) are views illustrating shapes of the lightoutput-side unit optical elements 24 and the back surface-side unitoptical elements 26 of the light guide plate 21 according to theembodiment. FIG. 5( a) illustrates a part of the light guide plate 21having the cross section illustrated in FIG. 4( b) in an enlargedmanner, and FIG. 5( b) illustrates a part of the light guide plate 21having the cross section illustrated in FIG. 4( a) in an enlargedmanner. As illustrated in FIG. 5( a), in the light output-side unitoptical elements 24, on a cross section (XZ cross section), which isparallel to a parallel-array direction thereof and perpendicular to athickness direction thereof, a cross-sectional shape thereof is atriangular shape, which has a base on one side surface of the baseportion 22 and has a protrusion shape protruding from the base portion22. In the light output-side unit optical elements 24 of thisembodiment, an example where a vertex opposed to each of the bases has acurved shape is illustrated. However, the light output-side unit opticalelements 24 may adopt a form of having not the curved shape but sharpcorner portions, and moreover, the bases thereof may be curved. Asillustrated in FIG. 5( a), in the light output-side unit opticalelements 24, a parallel pitch thereof is Pa, a width of the base portion22 side in the parallel-array direction (that is, a length of the baseof the triangular shape in cross section) is Wa, a height (dimension inthe thickness direction) of the light output-side unit optical elements24 is Ha, a vertex angle of the triangular shape in cross section is θ3, and angles other than the vertex angle are θ1 and θ2 . Typically, theparallel pitch Pa is equal to the length Wa of the base.

It is preferred that the cross-sectional shape of the light output-sideunit optical elements 24, which is illustrated in FIG. 4( b) and FIG. 5(a), satisfy at least one of the following condition A and condition B.

Condition A: the angles θ1 and θ2 of base angles located on the baseportion 22 triangular in cross section, which serve as angles other thanthe vertex angle θ3 , are 25° or more to 45° or less.

Condition B: A ratio (Ha/Wa) of a height Ha of the base to the length Wais 0.2 or more to 0.5 or less.

In a case where at least one of the condition A and the condition B issatisfied, among the light output from the light guide plate 21,components along the parallel-array direction (X direction) of the lightoutput-side unit optical elements 24 can be enhanced in condensingfunction in the normal direction of the light output surface 21 d of thelight guide plate 21 while allowing the light guide plate 21 to havepolarization property. As a result, in polarized light (firstdirectivity light L1: described later) output from the light guideplate, a ratio of a polarized light component that oscillates in apredetermined plane can be increased.

On the cross section (cross section along the direction where the lightoutput-side unit optical elements 24 are arrayed in parallel to oneanother) that appears in FIG. 4( b) and FIG. 5( a), it is preferred thatthe light output-side unit optical elements 24 of this embodiment havean isosceles triangular shape, and that the angles θ1 and θ2 be equal toeach other. By adopting such a form as described above, the brightnessin the front direction can be effectively increased, and symmetry can beimparted to an angle distribution of the brightness in a plane along theparallel-array direction (X direction) of the light output-side unitoptical elements 24.

Note that, the “triangular shape” in this specification includes notonly a triangular shape in a strict sense but also an approximatelytriangular shape including limitations in the manufacturing technology,an error at a molding time, and the like. Moreover, in a similar manner,terms, which are used in this specification and specify other shapes andgeometrical conditions, for example, terms such as “ellipsoid” and“circle” are not restricted to strict senses thereof, and areinterpreted to include errors to an extent that similar opticalfunctions can be expected.

As illustrated in FIG. 1, on the back surface side (Z1 side) of thelight guide plate 21, the back surface-side unit optical element portion25 is formed. On the back surface-side unit optical element portion 25,a plurality of the back surface-side unit optical elements 26 are formedto be arrayed in parallel to one another. The back surface-side unitoptical elements 26 are columnar, maintain a cross-sectional shape thatappears on a cross section illustrated in FIG. 4( a) and FIG. 5( b), anddefine the direction (X direction) perpendicular to the light guidingdirection of the light in the light guide plate as a longitudinaldirection. The plurality of back surface-side unit optical elements 26are arrayed in parallel to one another in the light guiding direction (Ydirection) of the light in the light guide plate. An array direction ofthe back surface-side unit optical elements 26 is substantially parallelto the transmission axis of the above-mentioned second polarizing plate14. As illustrated in FIG. 5( b), in each of the back surface-side unitoptical elements 26, on a cross section (YZ plane), which isapproximately parallel to a parallel-array direction (Y direction)thereof and perpendicular to a thickness direction (Z direction)thereof, a cross-sectional shape thereof is a triangular shape (wedgeshape), which has a base on the back surface-side (Z1 side) surface ofthe base portion 22 and has a protrusion shape protruding from the baseportion 22 to the back surface side (Z1 side). In the back surface-sideunit optical elements 26 of this embodiment, an example where a vertexof each thereof has a corner with an obtuse angle is described. However,the back surface-side unit optical elements 26 are not limited to this,and for example, an apex thereof may have a curved shape protruding tothe back surface side.

As illustrated in FIG. 5( b), the back surface-side unit opticalelements 26 have a parallel pitch of Pb, a width (that is, a length ofthe base of the triangular shape in cross section) of the base portion22 side in the parallel-array direction of Wb, a height (dimension inthe thickness direction) of the back surface-side unit optical elements26 of Hb, a vertex angle of the triangular shape in cross section of θ6,and angles other than the vertex angle of θ4 and θ5. This parallel pitchPb is equal to the length Wb of the base. A cross-sectional shape of theback surface-side unit optical elements 26 may be a symmetrical shape oran asymmetrical shape on the cross section, which is parallel to thearray direction and parallel to the thickness direction. FIG. 5( b)illustrates the cross-sectional shape of the back surface-side unitoptical elements 26 for use in the dual lamp-type surface light sourcedevice. In this case, it is preferred that the cross-sectional shape bea symmetrical shape on the cross section, which is parallel to the arraydirection and parallel to the thickness direction. More specifically,the cross-sectional shape of the back surface-side unit optical elements26 illustrated in FIG. 5( b) is an isosceles triangular shape, and thebase angles θ4 and θ5 are set equal to each other. Meanwhile, in a casewhere the back surface-side unit optical elements 26 are used for thesingle lamp-type surface light source device, the cross-sectional shapeof the back surface-side unit optical elements 26 may be set, forexample, to be an asymmetrical triangular shape as illustrated in FIG.6( b) to be described later. In this case, it is preferred for the baseangles θ4 and θ5 that the base angle located on the light source unit 10side in the array direction of the back surface-side unit opticalelements 26 become larger than the other base angle from a viewpoint ofefficiently guiding and outputting light. The back surface-side unitoptical elements 26 as described above are provided so that the lightfrom the light source unit 10 can be efficiently guided and output inthe light guide plate 21, and evenness in brightness in the plane alongthe parallel-array direction (Y direction) of the back surface-side unitoptical elements 26 and the like can be enhanced. Moreover, a diffusionfunction received by the light output from the light guide plate 21 canbe reduced as much as possible.

An example of dimensions of the respective portions of the light guideplate 21 is described below.

In the light output-side unit optical elements 24, the width Wa of abottom portion thereof may be set at 20 μm to 500 μm, and the height Hamay be set at 4 μm to 250 μm or less. Moreover, the vertex angle θ3 ofthe light output-side unit optical elements 24 may be set at 90° to 125°or less.

A thickness of the base portion 22 may be set at 0.25 mm to 10 mm, andan entire thickness of the light guide plate 21 may be set at 0.3 mm to10 mm.

In the back surface-side unit optical elements 26, the with Wb of abottom portion thereof may be set at 20 μm to 500 μm, and the height Hbmay be set at 1 μm to 10 μm. Moreover, the vertex angle θ6 of the backsurface-side unit optical elements 26 may be set at 176.0° to 179.6°.

The light guide plate 21 is capable of manufacturing, by, for example,extrusion, or shaping the light output-side unit optical elements 24 andthe back surface-side unit optical elements 26 on a base serving as thebase portion 22, the base portion 22 integrally with the lightoutput-side unit optical element portion 23 and the back surface-sideunit optical element portion 25. In a case of manufacturing the lightguide plate 21 by the extrusion, the light output-side unit opticalelement portion 23 and the back surface-side unit optical elementportion 25 may use the same resin material as a material serving as aparent material of the base portion 22, or may use a different materialtherefrom.

As the material serving as the parent material of the base portion 22 ofthe light guide plate 21, and as a material for forming the lightoutput-side unit optical elements 24 and the back surface-side unitoptical elements 26, various materials can be used as long as thematerials can transmit light therethrough efficiently. For example,materials, which are available inexpensively as well as are widely usedfor optical use and have excellent mechanical property, opticalproperty, stability, processability, and the like, can be used, and atransparent resin that contains, as amain component, one or more of anacrylic resin such as polymethyl methacrylate (PMMA), a styrene resin, apolycarbonate (PC) resin, polyethylene terephthalate (PET) resin,acrylonitrile, and the like, epoxy acrylate-based and urethaneacrylate-based reactive resins (ionizing radiation curable resin and thelike), glass, and the like can be used.

As illustrated in FIG. 1 and FIG. 4( a), the light source unit 10 isarranged at a position opposed to one surface or at positions opposed toboth surfaces, the surfaces being a pair of the side surfaces 21 a and21 b, which serve as both ends of the light output-side unit opticalelements 24 in the longitudinal direction (Ydirection), among two pairsof the plate-like side surfaces of the base portion 22 of the lightguide plate 21, each pair including the side surfaces opposed to eachother. The light source unit 10 is arranged along that surface. In thisembodiment, there is described an example where, as illustrated in FIG.1 and FIG. 4( a), the light source unit 10 is provided at positions,which face to the two side surfaces 21 a and 21 b of the light guideplate 21, along the side surfaces 21 a and 21 b. It is preferred thatthis light source unit 10 be a light emitting source such as a lightemitting diode (LED), which emits light with high directivity. The lightsource unit 10 of this embodiment includes a plurality of point lightsources 10 a arrayed therein, and the point light sources 10 a are LEDs.This light source unit 10 is adjustable in an output of each of thepoint light sources (LEDs) 10 a, that is, turning on and off of each ofthe point light sources 10 a, brightness thereof at a turning-on time,and the like by a control device (not shown) independently of outputs ofother point light sources.

On the back surface side of the light guide plate 21, the reflectionsheet 11 is provided. This reflection sheet 11 has a function to reflectthe light output from the back surface side and the like of the lightguide plate 21 and to return the light into the light guide plate 21. Asthis reflection sheet 11, for example, there can be used a sheet formedof a material such as metal, which has a high reflectance (for example,a regular-reflective silver foil sheet, one formed by depositingaluminum and the like on a thin metal plate), a sheet including, as afront surface layer, a thin film (for example, a metal thin film) formedof a material having a high reflectance (for example, a sheet formed bydepositing silver on a PET base), a sheet having a specular reflectivityby stacking a large number of two or more types of thin films differentin refractive index, a white foamed PET (polyethylene terephthalate)sheet having a diffusion reflectivity, and the like. From a viewpoint ofenhancing the light condensing property and the utilization efficiencyof the light, it is preferred to use a reflection sheet, which enablesthe so-called specular reflection, such as the sheet formed of thematerial such as metal, which has a high reflectance, and the sheetincluding, as the front surface layer, the thin film (for example, ametal thin film) formed of the material having a high reflectance. It isestimated that the reflection sheet, which enables the specularreflection, allows the light to perform the specular reflection so thatthe directivity of the light is not lost, and as a result, thepolarization direction of the output light is maintained. Accordingly,the reflection sheet 11 can also contribute to realization of a desireddistribution of the output light.

FIGS. 6( a) and 6(b) are views illustrating states of output light fromthe light guide plate 21 and the prism sheet 30. FIG. 6( a) is a viewillustrating a case of the above-mentioned dual lamp-type surface lightsource device, and FIG. 6( b) is a view illustrating a single lamp-typesurface light source device for reference. The light guide plate 21 hassuch a configuration as described above, and the light output from thelight output surface 21 d (surface on the prism sheet 30 side) becomeslight, which has directivity having maximum intensity in a predetermineddirection, and has a predetermined half width (this light is hereinafterreferred to as first directivity light L1 in some cases). In FIG. 6( a),the light source unit 10 is arranged on the side surfaces 21 a and 21 bof the light guide plate 21, and a main light guiding direction of thelight from the light source unit 10 is the Y direction. In this case,the light guide plate 21 has such a configuration as described above,and hence an output direction and a polarization state of the light thatpropagates through the light guide plate 21 are controlled by functionsto be described later. As a result, as illustrated in FIG. 6( a), thelight output from the light guide plate 21 becomes polarized light thathas the maximum intensity (peak) in a direction (hereinafter referred toas a first direction in some cases) that forms an angle α toward theside surface 21 b side with respect to the normal direction of the lightoutput surface 21 d on the YZ plane. In the example of the figure, theangle α of this embodiment is approximately 73°. The light guide plateis designed as appropriate so that any appropriate angle α can berealized depending on the purpose. For example, the angle α may be 65°to 80°. Note that, regardless of whether the light guide plate 21 foruse in the present invention is the single lamp-type light guide plateor the dual lamp-type light guide plate, the control for the outputdirection and the polarization state can be realized favorably.

Moreover, the light guide plate 21 of this embodiment hascharacteristics of outputting polarized light in which a ratio of apolarized light component that oscillates in the plane (in the YZ plane)in a direction parallel to the light guiding direction of the light ishigh. That is, the first directivity light becomes polarized light inwhich the ratio of the polarized light component that oscillates in theYZ plane is high. In the following, in some cases, the polarized lightcomponent that oscillates in the YZ plane is referred to as a Pcomponent, and a polarized light component that oscillates in a plane(XY plane), which is parallel to the light guiding direction of thelight and perpendicular to the YZ plane, is referred to as an Scomponent. Accordingly, the polarization direction (oscillationdirection) of the P component becomes approximately parallel to thetransmission axis direction (Y direction) of the second polarizing plate14. As described later, the prism sheet 30 outputs second directivitylight, which has maximum intensity in the second direction (normaldirection), while maintaining the polarization state of the firstdirectivity light, and accordingly, the second directivity light alsobecomes polarized light in which the ratio of the P component is high.As a result, the light absorbed by the second polarizing plate can bereduced, and accordingly, the bright liquid crystal display apparatus,in which the utilization efficiency of the light is high, can beobtained.

Note that, a principle that the light guide plate 21 guides the lightuses a phenomenon that the light causes the total reflection when anincident angle θa reaches θc in the following Expression 1 on aninterface between mediums that are optically dense (refractive index n1)and thin (refractive index n2), and θc refers to a critical angle.sin θc=n2/n1  (Expression 1)

The light guided in the light guide plate 21 is output from the lightguide plate 21 when the incident angle θa with respect to the lightoutput surface 21 d by the total reflection in the back surface-sideunit optical elements 26 becomes smaller than this critical angle θc.

FIGS. 7( a) to 7(c) are graphs showing various relationships among theincident angle, the P component, and the S component. As shown in FIG.7( a), in a region where the incident angle is slightly smaller than thecritical angle, in light of the P component and light of the Scomponent, a reflectance of the light of the P component becomes smallerthan a reflectance of the light of the S component. Accordingly, thelight reflected in the inside of the light output surface 21 d of thelight guide plate 21 may be set to be polarized light in which a ratioof the S component is high, and the light output from the light outputsurface 21 d may be set to be polarized light in which a ratio of the Pcomponent is high. For example, when n1=1.5 and n2=1.0, in a case whereθa=33° 41′24″, the reflectance of the P component becomes 0, the lightof the P component is output from the light output surface of the lightguide plate 21, and the reflected light may be set to be polarized lightthat includes only the S component. As a result, polarized light inwhich an amount of the P component is large is output from the lightoutput surface 21 d of the light guide plate 21.

In this embodiment, the refractive index of the light guide plate 21 andthe base angles θ4 and θ5 of the back surface-side unit optical elements26 are set so that the incident angle ea with respect to the lightoutput surface 21 d may be slightly smaller than the critical angle θc.By adopting such a form, the light output from the light guide plate 21is output as the polarized light in which the amount of P component islarge. In addition, the incident angle θa is set in the specific smallregion, and accordingly, a light output angle is also limited to aspecific small region. That is, the polarized light in which the ratioof the P component is high, and having the maximum intensity in thefirst direction (direction of an output angle α), can be output as thefirst directivity light L1 from the light output surface 21 d.

Such light (first directivity light L1) output from the light guideplate 21 contains the P component, preferably by 52% or more, morepreferably 55% or more. The first directivity light L1 has such propertyso that the light absorbed by the second polarizing plate can bereduced. Thus the bright liquid crystal display apparatus, in which theutilization efficiency of the light is high, can be obtained. Note that,an upper limit of the ratio of the P component is ideally 100%, is 60%in one embodiment, and is 57% in another embodiment.

Next, a description is made of the prism sheet 30. As illustrated inFIG. 1 and FIGS. 4( a) and 4(b), the prism sheet 30 includes a baseportion 31 formed into a sheet shape, and a prism portion 32 provided ona surface (light incident surface) of the base portion 31 on the lightguide plate 21 side (Z1 side). This prism sheet 30 converts the firstdirectivity light L1, which enters from the light guide plate 21 and hasthe maximum intensity in the first direction, into the seconddirectivity light L2, which has maximum intensity in the seconddirection that is an approximately normal direction (where the angle ρin FIGS. 6( a) and 6(b) is approximately 90°) of a light output surface30 a of the prism sheet 30, by a total reflection in an inside of eachof unit prisms 33, and the like while maintaining the polarization stateof the first directivity light L1. Then, the prism sheet 30 outputs thesecond directivity light L2 from the light output surface 30 a. Asdescribed above, the “first directivity light having the maximumintensity in the first direction” means light that has an intensitydistribution in which a peak of the maximum intensity of the intensitydistribution of the brightness is located in the first direction, and inthis case, the first directivity light corresponds to the light outputfrom the light guide plate 21. Moreover, the “approximately normaldirection” includes directions within a predetermined angle from thenormal direction, for example, directions within a range of ±10° fromthe normal direction.

The base portion 31 is a sheet-like member, in which the prism portion32 is integrally formed on the light guide plate 21 side (Z1 side), andis a member that becomes a support of the prism portion 32. A surface ofthis base portion 31 on an opposite side (Z2 side) to the light guideplate 21 side becomes the light output surface 30 a. The light outputsurface 30 a of this embodiment is formed as a flat and smooth surface.The base portion 31 is a member with a film shape or the like, which isused for a surface light source device mechanism of a usual opticaldisplay or liquid crystal display.

It is preferred that the base portion 31 substantially have opticalisotropy. In this specification, “substantially have optical isotropy”refers to that a retardation value is small to an extent of notsubstantially affecting the optical characteristics of the liquidcrystal display apparatus. For example, an in-plane retardation Re ofthe base portion 31 is preferably 20 nm or less, more preferably 10 nmor less. When the in-plane retardation remains within such a range, thefirst directivity light output from the light guide plate can be outputas the second directivity light in a predetermined direction withoutsubstantially changing the polarization state of the first directivitylight (while maintaining the ratio of the P component). Note that, thein-plane retardation Re is a retardation value in the plane, which ismeasured by light with a wavelength of 590 nm at 23° C. The in-planeretardation Re is represented by Re=(nx−ny)×d. In this case, nx is arefractive index in a direction where the refractive index becomesmaximum in a plane of an optical member (that is, the direction is theslow axis direction), ny is a refractive index in a directionperpendicular to the slow axis direction in the plane (that is, thedirection is the fast axis direction), and d is a thickness (nm) of theoptical member.

In another embodiment of the present invention, the base portion 31 mayhave the in-plane retardation value. The in-plane retardation Re of thebase portion 31 differs significantly depending on a thickness thereof.The in-plane retardation Re is 100 nm to 10,000 nm, for example. In thisembodiment, the base portion 31 only needs to be arranged so that theslow axis thereof and the transmission axis of the second polarizingplate can be perpendicular or parallel to each other.

Moreover, a photoelastic coefficient of the base portion 31 ispreferably −10×10⁻¹² m²/N to 10×10⁻¹² m²/N, more preferably −5×10⁻¹²m²/N to 5×10⁻¹² m²/N, still more preferably, −3×10⁻¹² m²/N to 3×10⁻¹²m²/N. When the photoelastic coefficient remains within such a range,there is an advantage in that the in-plane retardation is hardlyincreased even when a stress due to a volume change of the base portionis generated in a temperature range (0° C. to 50° C.) and a humidityrange (0% to 90%) at which the liquid crystal display apparatus isassumed to be used in general, and moreover, the in-plane retardation ishardly increased in a similar manner even when a stress caused after thebase portion is fixed and attached by a general method is applied,thereby obtaining stable characteristics of the surface light sourcedevice.

As a material for forming the base portion 31, it is preferred to use acolorless and transparent material having transmission performance inthe entire visible light wavelength range. Moreover, in a case offorming the prism on the base portion 31 by using the ionizing radiationcurable resin, it is preferred to use a material further having ionizingradiation transmission property. For example, it is preferred to use afilm formed of TAC (cellulose triacetate), an acrylic resin such asPMMA, or a PC resin, and it is more preferred to use an unstretched filmfrom a viewpoint of imparting the optical isotropy. Moreover, it ispreferred that a thickness of the base portion 31 be 25 μm to 300 μm interms of handling easiness and strength thereof. Note that, the ionizingradiation means a radiation such as ultraviolet rays and electron beams,which has an energy quantum capable of crosslinking or polymerizingmolecules.

As illustrated in FIG. 1 and FIGS. 4( a) and 4(b), in the prism portion32, on a surface of the base portion 31 on the light incident side (Z1side), a plurality of the unit prisms 33 are formed to be arrayed inparallel along a sheet surface thereof. The plurality of unit prisms 33are columnar, define, as a longitudinal direction, a direction (Xdirection) perpendicular to the light guiding direction of the light inthe light guide plate, are extended in the longitudinal direction whilemaintaining a predetermined cross-sectional shape, and are arrayed inparallel in the light guiding direction (Y direction) of the light inthe light guide plate. In this case, in each of optical sheets and thelike, the sheet surface refers to a surface along a plane direction of asheet when the entire sheet is viewed. The sheet surface is used as thesame definition in this specification and the claims. For example, thesheet surface of the prism sheet 30 is a surface along a plane directionof the prism sheet 30 when the entire prism sheet 30 is viewed. Thesheet surface of the prism sheet 30 is a surface parallel to the lightoutput surface 30 a of the prism sheet 30, and is a surfacesubstantially parallel to the viewing screen of the liquid crystaldisplay panel 15.

The longitudinal direction (ridge line direction) of the unit prisms 33may be directed to such a direction approximately perpendicular to thetransmission axis of the second polarizing plate 14 of the liquidcrystal display panel 15 when the display apparatus 1 is viewed from thefront direction (Z direction). That is, on the surface parallel to thedisplay surface of the display apparatus 1, a parallel-array directionof the unit prisms 33 may be set approximately parallel to thetransmission axis of the second polarizing plate 14 of the liquidcrystal display panel 15. Moreover, at this time, the longitudinaldirection (ridge line direction) of the unit prisms 33 is approximatelyperpendicular to the longitudinal direction (ridge line direction) ofthe light output-side unit optical elements 24 of the light guide plate21 when the display apparatus 1 is viewed from the front direction (Zdirection).

Note that, as described above, the ridge line directions and/or theaxial directions of the respective members in the liquid crystal displayapparatus of this embodiment are typically approximately perpendicularor approximately parallel to each other. However, in some cases, therespective members interfere with each other to generate moire dependingon a matrix of the liquid crystal layer and on a pitch and array of theunit optical elements of the prism sheet or the light guide plate. Inthat case, the ridge line direction of the unit prisms 33 and/or theridge line directions of the light output-side unit optical elements 24and/or the back surface-side unit optical elements 26 of the light guideplate 21 are arranged obliquely within a predetermined range when thedisplay apparatus 1 is viewed from the front direction (Z direction) sothat it is possible to avoid the moire. The range of such obliquearrangement is preferably 20° or less, more preferably 5° or less. Whenthe arrangement falls over this range, directivity of the light, whichis described later, is affected in some cases.

FIG. 8 is a view illustrating the prism portion 32 of the prism sheet 30of this embodiment. FIG. 8 is a view enlargedly illustrating a part ofthe cross section illustrated in FIG. 4( a). As illustrated in FIG. 8,the unit prism 33 of this embodiment has a shape protruding from asurface of the base portion 31 on the light guide plate 21 side to thelight guide plate 21 side (Z1 side), and a width of the unit prism 33 ina direction parallel to the sheet surface of the base portion 31 isbecoming smaller as being spaced apart from the base portion 31 alongthe normal direction (Z direction) of the base portion 31.

As illustrated in FIG. 8, the unit prism 33 of this embodiment is aso-called triangular column prism, in which a cross-sectional shape on across section parallel to the array direction (Y direction) and parallelto the thickness direction (Z direction) is a triangular shape. Thecross-sectional shape of the unit prism 33 illustrated in FIG. 8 is ascalene triangle in which a first inclined surface 34 located on thelight source unit 10 side in the array direction of the unit prism 33 isformed as a steeper inclined surface than a second inclined surface 35on the other side. At this time, when an angle (incident surface angle)formed by the first inclined surface 34 and a normal F of the sheetsurface of the prism sheet 30 is defined as φ1, and an angle (reflectingsurface angle) formed by the second inclined surface 35 and the normal Fof the sheet surface of the prism sheet 30 is defined as φ2, φ1<φ2. Thisis in order to direct the first directivity light L1, which is outputfrom the light guide plate 21 while having a peak in the firstdirection, toward an approximately normal direction (second direction)of the light output surface 30 a.

A pitch of this unit prism 33 is P, and a width thereof on a baseportion 31 side in the cross-sectional shape is W. The pitch P of thisembodiment is equal to the width W. Moreover, a height of the unit prism33 (that is, a dimension from a point that is a root between the unitprisms 33 in the thickness direction to a vertex t) is H.

A description is made below of a behavior of the light that enters theunit prism 33. Note that, in FIG. 8 and FIG. 9 to be described later,for description convenience, as the behavior of the light,representative light beams corresponding to the respective components ofthe light are indicated by the arrows, and aspect ratios, dimensionalratios between the respective layers, and the like are appropriatelychanged from those of the actual dimensions and are illustratedexaggeratedly.

The first directivity light L1, which is output from the light guideplate 21 and has the maximum intensity in the first direction, travelsstraight through an air layer (refractive index: approximately 1.0),thereafter enters the first inclined surface 34 of the unit prism 33,travels approximately straight through the unit prism 33, is totallyreflected on the second inclined surface 35, and is output as the seconddirectivity light L2 having the maximum intensity in the direction(second direction) approximately perpendicular to the light outputsurface 30 a (sheet surface) in the array direction of the unit prism33. At this time, a bias of the polarization direction in the firstdirectivity light L1 is also maintained in the second directivity lightL2. Accordingly, it becomes possible to impart directivity, which isstrong in the normal direction of the sheet surface, to the lightreflected on the second inclined surface 35, and in comparison with acase where the directivity as described above is not imparted,absorption of the light by the black matrix of the liquid crystaldisplay panel 15 is suppressed, and the utilization efficiency of thelight can be enhanced. Moreover, even the strong directivity is impartedto the light, the polarization direction of the light is not varied.Further, the first inclined surface 34 and the second inclined surface35 include flat surfaces. Thus, it becomes easy to ensure shape accuracythereof, and accordingly, quality control thereof is easy, and the massproductivity can be enhanced.

An inclination angle of the first inclined surface 34 of the unit prism33 illustrated in FIG. 8 is appropriately adjusted depending on thedirection (first direction; output angle: a) where the first directivitylight L1 has the maximum intensity. In general, the angle φ1 formed bythe first inclined surface 34 and the normal F of the light outputsurface 30 a (sheet surface) of the prism sheet 30 is 30° to 37°.Moreover, an inclination angle of each flat surface of the secondinclined surface 35 is adjusted so that the first directivity light L1can be converted into, by internal reflection thereof, the seconddirectivity light L2 having the maximum intensity in the normaldirection of the light output surface 30 a (sheet surface) of the prismsheet 30. The angle φ2 formed by each flat surface of the secondinclined surface 35 with the normal F is appropriately adjusteddepending on a predetermined direction where the first directivity lightL1 has the maximum intensity, and in usual, is 30° to 37°, andpreferably satisfy φ2>φ1. The height H of the unit prism 33 is changeddepending on the pitch P of the unit prism 33. However, in a case wherethe pitch P is 50 μm, the height H is 30 μm to 45 μm in usual. The pitchP of the unit prism 33 is not particularly limited, but the pitch P is10 μm to 100 μm in usual.

The vertex t of the unit prism 33 may have a sharp shape as illustratedin FIG. 8, or though not illustrated, may be formed into a curved shapein which a vicinity of the vertex t is chamfered, or a tip end thereofmay be cut so as to become a flat surface. In a case where the tip endof the vertex t of the unit prism 33 is cut, the height H of the unitprism 33 is defined to be a height from the point that is the rootbetween the unit prisms 33 in the thickness direction to a flat surfaceof the tip end.

FIG. 9 is a view illustrating a unit prism 33 according to anotherembodiment. FIG. 9 illustrates a shape of the unit prism 33 on a similarcross section to that of FIG. 8. As illustrated in FIG. 9, the unitprism 33 may adopt a form in which the second inclined surface 35 has aplurality of flat surfaces 35 a and 35 b different in inclination angle.The respective flat surfaces 35 a and 35 b of the second inclinedsurface 35 have inclination angles, which allow the first directivitylight L1 (L1 a, L1 b) that enters from the first inclined surface 34 tobe internally reflected so as to be converted into the seconddirectivity light L2 (L2 a, L2 b) having the maximum intensity in theapproximately normal direction with respect to the light output surface30 a of the prism sheet 30 for each of components that reach therespective flat surfaces. The inclination angles are controllableindividually for each of the flat surfaces. As illustrated in FIG. 9,between the respective flat surfaces of the second incline surface 35,an angle (first reflecting surface angle) formed by the flat surface 35a on the vertex t side (Z1 side) and the normal F is φ2, and an angle(second reflecting surface angle) formed by the flat surface 35 b on thebase portion 31 side (Z2 side) of the second inclined surface 35 and thenormal F is φ3.

The first directivity light L1 (L1 a, L1 b), which is output from thelight guide plate 21 and has the maximum intensity in the firstdirection, travels straight through the air layer (refractive index:approximately 1.0), thereafter enters the first inclined surface 34 ofthe unit prism 33, travels approximately straight through the unit prism33, is individually reflected on the flat surfaces 35 a and 35 b of thesecond inclined surface 35, and for each of the components that reachthe individual flat surfaces 35 a and 35 b, is output as the seconddirectivity light L2 (L2 a, L2 b) having the maximum intensity in thedirection (second direction) perpendicular to the light output surface30 a (sheet surface) in the array direction of the unit prism 33. Notethat, the first directivity light L1 is blocked by the adjacent unitprism 33. Therefore, in the respective flat surfaces of the secondinclined surface 35, as the flat surface is closer to the base portion31 side (Z2 side), only such a component of the first directivity lightL1 that the angle formed with the normal of the sheet surface is smallreaches the flat surface. In the embodiment of FIG. 9, the firstdirectivity light L1 is illustrated by being classified into L1 a and L1b for each of the components that reach the individual flat surfaces ofthe second inclined surface 35. The first directivity light L1 is lightformed by synthesizing the respective light components (L1 a, L1 billustrated in FIG. 9), which are output from the light guide plate 21,with each other. Accordingly, in a case where the unit prism 33 asillustrated in FIG. 9 is formed, the directivity of the seconddirectivity light L2 can be further intensified.

Accordingly, also in such a case where the unit prism 33 has the form asillustrated in FIG. 9, it becomes possible to impart the directivity,which is strong in the normal direction of the sheet surface, to thelight (output light from the light output surface of the prism sheet 30)formed by synthesizing the respective light components, which arereflected from the respective flat surfaces 35 a and 35 b, with eachother, and the polarization direction of the light is not varied,either. Moreover, even in the form as illustrated in FIG. 9, the firstinclined surface 34 and the second inclined surface 35 include the flatsurfaces. Thus, it becomes easy to ensure the shape accuracy thereof,and accordingly, the quality control is easy, and the mass productivitycan be enhanced.

In the form illustrated in FIG. 9, the inclination angles of therespective flat surfaces of the second inclined surface 35 are adjustedindividually for each of the flat surfaces so that the first directivitylight L1 can be converted into the second directivity light L2, whichhas the maximum intensity in the normal direction of the light outputsurface 30 a (sheet surface) of the prism sheet 30, by the internalreflection. It is preferred for the inclination angles of the respectiveflat surfaces of the second inclined surface 35 that the flat surfacecloser to the vertex t of the unit prism 33 have a larger angle formedwith the normal F with respect to the light output surface 30 a (sheetsurface) of the prism sheet 30. That is, in the case of the unit prism33 illustrated in FIG. 9, it is preferred to satisfy φ2>φ3. In such amanner, the peak of the maximum intensity of the second directivitylight L2 is further narrowed, the directivity of the second directivitylight L2 can be enhanced, and the brightness in the front direction canbe enhanced. Moreover, the angles φ2 and φ3, which are formed by therespective flat surfaces of the second inclined surfaces 35 with thenormal F, are appropriately adjusted depending on the predetermineddirection where the first directivity light L1 has the maximumintensity, and are 30° to 37° in usual.

As illustrated in FIG. 9, in the case where the second inclined surface35 of the unit prism 33 includes the two flat surfaces 35 a and 35 b, aposition where a boundary point between the respective flat surfaces 35a and 35 b, at which the inclination angle of the second inclinedsurface 35 is changed, is provided is appropriately adjusted dependingon the directivity direction of the first directivity light. When theheight H of the unit prism 33 is defined to be 100%, this boundary pointis provided at a position where a height from a basal surface (surfacewhere a point serving as the root between the unit prisms 33 is located)of the unit prism 33 is 20% to 80%.

Note that, in the case where the second inclined surface 35 includes theplurality of flat surfaces, the number of flat surfaces is not limitedto the number of those illustrated, and the unit prism 33 may includethree or more flat surfaces.

In usual, because of excellence in the shape accuracy and excellence inthe mass productivity, the prism sheet 30 adopts a form, in which asheet-like member having light transmissivity is used as the baseportion 31, and the prism portion 32 is provided on the one-side surfacethereof. However, the prism sheet 30 may have a single layerconfiguration formed of a single layer by the extrusion of a singlematerial or the like. Similar materials can be used as a material forforming the prism portion in the case of manufacturing the prism sheet30 by providing the prism portion 32 on the one-side surface side of thebase portion having the light transmissivity, and as a material forforming the optical sheet in the case of manufacturing the prism sheet30 with the single layer configuration by performing the extrusion ofthe single material. In the following, the material for forming theprism portion and the material for forming the prism sheet with thesingle layer configuration are generically referred to as a prismmaterial. For example, in a case of using the epoxy acrylate-based or aurethane acrylate-based reactive resin (ionizing radiation curable resinor the like), it is possible to mold the prism material by the 2Pmethod, and the prism portion can be molded on the base, or by curingthe material alone in a die. Moreover, in the case of forming the prismsheet by the extrusion, as the prism material, there can be used alight-transmissive thermoplastic resin such as a polyester resin such asPC and PET, an acrylic resin such as PMMA and MS, and cyclic polyolefin.Note that, in the case of forming the prism sheet by the extrusion,molecules of the resin are aligned and the birefringence is generateddepending on forming conditions thereof, and accordingly, it ispreferred that the prism sheet be formed under such conditions that donot allow the molecules to be aligned.

As a manufacturing method of the prism sheet 30, methods heretoforeknown in public can be appropriately used. For example, the prism sheet30 may be formed in such a manner that the material for forming theprism portion, such as an ultraviolet curable resin, is put into ashaping mold for the prism portion 32 having a desired unit prism shape,a base serving as the base portion 31 is stacked thereon, ultravioletrays and the like are radiated while bringing the base into pressurecontact with the material for forming the prism columns by using alaminator and the like so that the material for forming the prismportion is thereby cured, and the mold for the prism columns is releasedor removed (for example, refer to FIG. 2 of JP 2009-37204 A). Moreover,the prism sheet 30 can be manufactured continuously when a liquidmaterial for forming the prism portion is applied and filled onto arotating roll intaglio having recessed portions with a shape reverse tothe prism shape, the member serving as the base portion 31 is suppliedthereto and is pressed against the roll intaglio from above the liquidmaterial for forming the prism portion on a printing plate, in such apressed state, the liquid material for forming the prism portion iscured by irradiation of the ultraviolet rays and the like, andthereafter, the cured liquid material for forming the prism portion isreleased from the rotating roll intaglio together with the base (forexample, refer to JP 05-169015 A). Moreover, it is possible tomanufacture the prism sheet 30 also by the extrusion method by using thethermoplastic resin as described above. As a material in the event ofperforming the extrusion for the prism sheet 30, the material forforming the prism sheet described above can be used.

A description is made of a method of controlling the polarizationdirection in the prism sheet 30 and an effect thereof. As illustrated inFIGS. 6 (a) and 6 (b), the first directivity light L1, which is outputfrom the light guide plate 21 and has the maximum intensity in the firstdirection, is output as the second directivity light L2, which has themaximum intensity in the second direction (normal direction (outputangle: 0°; angle β: 90°) of the liquid crystal display panel 15), by thetotal reflection or the like on the second inclined surface 35 of theunit prism 33 of the prism sheet 30. At this time, for example, in acase where the refractive index n1 of the prism portion 32 is 1.50, θcbecomes 41°48′37″ because the refractive index n2 of air is 1.0, and theincident light is totally reflected when the incident angle θb is equalto or larger than θc (θn≧θc). As shown in FIGS. 7( b) and 7(c), in thetotal reflection region (θb≧c), the light of the P component and thelight of the S component are output while differentiating and changingphases thereof depending on the incident angle θb. This fact affects thepolarization direction of the polarized light thus output. As a measuretherefor, the incident angle θb is controlled so that the polarizationdirection of the light that enters the liquid crystal display panel 15can be controlled, and the enhancement of the utilization efficiency ofthe light can be achieved. In this embodiment, the inclination anglesand refractive indices of the first inclined surface 34 and the secondinclined surface 35 of the unit prism 33 are controlled so that theincident angle θb is controlled. In this manner, in the total reflectionregion as shown in FIGS. 7( b) and 7(c), a retardation between the Pcomponent and the S component can be reduced, and the influence on thepolarization direction of the polarized light can be minimized. As aresult, the second directivity light can be output in the seconddirection (approximately normal direction) while substantiallymaintaining the polarization state of the first directivity light. Asdescribed above, the ratio of the P component in the first directivitylight is high, and accordingly, the polarization state thereof ismaintained so that the light absorbed by the second polarization plate14 can be reduced, and it becomes possible to effectively use theincident light onto the liquid crystal display panel 15.

FIGS. 10( a) and 10(b) are graphs showing an intensity distribution ofbrightness of the first directivity light L1 output from the light guideplate 21 of the embodiment and an intensity distribution of brightnessof the second directivity light L2 output from the prism sheet 30 of theembodiment. FIG. 10( a) is a graph showing an example of the intensitydistribution of the brightness in the first directivity light L1 outputfrom the light guide plate 21. FIG. 10( b) is a graph showing an exampleof the intensity distribution of the brightness in the seconddirectivity light L2 output from the light output surface 30 a of theprism sheet 30. For example, this graph is obtained by measuring theintensity distribution of the brightness of the light, which is outputfrom the light guide plate 21, at room temperature in the atmosphere, byusing a luminance meter provided with a goniometer or a lightdistribution measuring device such as EZ Contrast manufactured by ELDIMS. A.

As shown in FIG. 10( a), the first directivity light output from thelight guide plate 21 of the embodiment has the maximum intensity in adirection of approximately 73° to the side surface 21 b side (Y2 side)with respect to the normal of the light output surface of the lightguide plate 21 in the up-and-down direction (Y direction) of the screen,and is distributed in a range of 60° to 80°. Note that, it is preferredthat most of the first directivity light L1 be directed to angles formedwith normal in this range. However, light out of this range may bepresent. In the first directivity light L1, an angle (half width angle)at which a half width of the intensity distribution thereof is obtainedcan be set at ±5° or more, and in usual, is ±10° to 20°, and inaddition, the first directivity light L1 is polarized light in which aratio of light (P component) having such a polarization direction thathas an oscillation surface in the YZ plane is high. The half widthrefers to an angular difference from an angle having a maximum value,which is 100% in the peak of the maximum intensity of the brightness, toan angle when the intensity of the brightness becomes 50%. Thedirectivity is weakened as the half width is larger.

As shown in FIG. 10( b), the second directivity light L2, which is thelight output from the light output surface 30 a of the prism sheet 30,has a maximum intensity in the normal direction of the sheet surface bya deflection function of the unit prism 33, and a half width thereof canbe set smaller than the half width of the first directivity light L1.Moreover, by an optical function of the unit prism 33 of the prism sheet30 of this embodiment, the prism sheet 30 can convert the light, whichis output from the light guide plate 21, so that a half width angle ofthe light output from the light output surface 30 a can be ±20° or less,and further can be ±10° or less by adopting a more suitable form. In thelight output from the light output surface 30 a of the prism sheet 30,as the half width thereof is smaller, the brightness in the frontdirection is enhanced, and the variations of the polarization directiondue to expansion of the directivity are also reduced. As describedabove, the surface light source device 20 of this embodiment includesthe above-mentioned light guide plate 21 and prism sheet 30, and canthereby output light with such high directivity that allows the halfwidth angle to be ±20° or less, that is, approximately parallel lightfrom the light output surface (light output surface 30 a of the prismsheet 30) of the surface light source device 20 in the normal directionthereof. In addition, the surface light source device 20 can change theoutput light to the light in which the ratio of the light (P component)of such a polarization direction that has the oscillation surface in thedirection approximately parallel to the transmission axis of the secondpolarizing plate 14, that is, in the YZ plane is high. As a result, thelight absorbed by the second polarizing plate 14 can be reduced, and itbecomes possible to effectively use the incident light onto the liquidcrystal display panel 15.

FIGS. 11( a) to 11(d) are views illustrating relationships among thepolarization directions of the output light from the light guide plate21 and prism sheet 30 and the transmission axis of the first polarizingplate 13 and the transmission axis of the second polarizing plate 14 inthis embodiment. As described above, in the light (first directivitylight) output from the light guide plate 21, the ratio of the Pcomponent is high, and a main polarization direction thereof issubstantially an arrow D1 direction (Y direction) as illustrated in FIG.11( a). Moreover, the light output from the light guide plate 21 isoutput while an intensity peak direction thereof is deflected by theprism sheet 30. At this time, the output light is deflected by the totalreflection on the interface of the unit prism 33, and in addition, thebase portion 31 of the prism sheet 30 is a member that does not have thebirefringence. Accordingly, the polarization direction of the light(second directivity light) output from the prism sheet 30 issubstantially an arrow D2 direction (Y direction) as illustrated in FIG.11( b). That is, the light output from the surface light source device20 is polarized light mainly having a polarization direction as thearrow D2 direction.

The light output from the surface light source device 20 enters thesecond polarizing plate 14 of the liquid crystal display panel 15. Asillustrated in FIG. 11( c), the transmission axis of this secondpolarizing plate 14 is substantially an arrow D3 direction (Ydirection). The direction D3 of this transmission axis of the secondpolarizing plate 14 is a direction (Y direction) approximately parallelto the array direction of the back surface-side unit optical elements 26and the array direction of the unit prisms 33. Moreover, as illustratedin FIG. 11( d), the transmission axis of the first polarizing plate 13is substantially an arrow D4 direction (X direction). Accordingly, themain polarization direction D2 of the light output from the surfacelight source device 20 (prism sheet 30) and the transmission axis D3 ofthe second polarizing plate 14 are parallel to each other. Moreover, thetransmission axis D4 of the first polarizing plate 13 is perpendicularto the transmission axis D3 of the second polarizing plate 14, and isapproximately parallel to the polarization direction of the light inwhich the polarization direction is rotated by 90° by the liquid crystalcell 12 to which the electric field is applied. Further, in the lightthat enters the liquid crystal display panel 15, the half width thereofis narrower in comparison with that of the conventional technique, andthe directivity thereof becomes high. Accordingly, the variations andthe like of the polarization direction are small. Therefore, an amountof the light (polarized light), which is output from the surface lightsource device 20 (prism sheet 30) and is absorbed by the secondpolarizing plate 14, can be reduced to a large extent, and theutilization efficiency of the light is enhanced.

As described above, according to this embodiment, the output directionof the first directivity light L1, in which the ratio of the P componentin the polarized light output from the light guide plate 21 is high andwhich has the maximum intensity in the first direction, is deflected tothe second direction (front direction of the screen of the liquidcrystal display apparatus 1) by the prism sheet 30 so that the seconddirectivity light L2 is output as light, which maintains thepolarization state of the first directivity light L1, and includes alarge amount of the polarized light having the polarization directionparallel to the transmission axis of the second polarizing plate 14.Moreover, the transmission axis of the first polarizing plate 13 isperpendicular to the transmission axis of the second polarizing plate14, and is approximately parallel to the polarization direction of thelight in which the polarization direction is rotated by 90° by theliquid crystal cell 12 to which the electric field is applied.Therefore, the transmittance of the liquid crystal display panel 15 canbe maximized, the light utilization efficiency of the display apparatus1 can be enhanced, and a bright image can be displayed.

The description has been made above of the specific embodiment of thepresent invention. However, it is apparent for those skilled in the artthat various modifications can be made without departing from thetechnical idea of the present invention. The present inventionincorporates all of such modifications. A description is made below ofsome typical examples among the possible modifications. It is needlessto say that forms of the possible modifications described below andforms of modifications that are omitted from the description and areapparent for those skilled in the art may be combined with one anotheras appropriate.

(1) The prism sheet 30 may adopt a form including only the prism portion32 without including the base portion 31 when sufficient rigidity or thelike is inherent therein. Moreover, the prism sheet 30 is not limited tothe form of being a separate body from the second polarizing plate 14and the polarized light selective reflection sheet 16, and may adopt aform of integrating the prism portion 32 with the light guide plate 21side (Z1 side) of the second polarizing plate 14 or the polarized lightselective reflection sheet 16. When such a form is adopted, theinfluence of the base portion 31 on the polarization direction of thelight can be reduced to a large extent, and accordingly, a brighterliquid crystal display apparatus can be obtained. Moreover, the numberof components can be reduced, and hence the liquid crystal displayapparatus can be manufactured inexpensively, and the form thus adoptedcan contribute to the thinning of the liquid crystal display apparatus.The thinning of the liquid crystal display apparatus increases choicesof design, and accordingly, is commercially valuable. Further, when sucha form is adopted, the prism sheet can be avoided from being flawed bybeing rubbed when attaching the prism sheet onto the surface lightsource device (substantially, the light guide plate), and accordingly,turbidity of display, which is caused by such flaws, can be prevented.For example, a prism sheet-added polarizing plate in which the prismsheet is integrated with the polarizing plate can be used as the secondpolarizing plate. A description is made below of typical examples ofspecific characteristics, material and the like of polarizers usablesuitably for the prism sheet-added polarizing plate.

The transmittance of the above-mentioned polarizer (single axistransmittance) at the wavelength of 589 nm is preferably 41% or more,more preferably 42% or more. Note that, the theoretical upper limit ofthe single axis transmittance is 50%. In addition, polarization degreethereof is preferably from 99.5% to 100%, more preferably from 99.9% to100%. As long as the polarization degree falls within the range,contrast in the front direction can be further higher when using theliquid crystal display apparatus.

The single axis transmittance and polarization degree described abovecan be measured with a spectrophotometer. A specific measurement methodfor the polarization degree described above may involve measuringparallel transmittance (H₀) and perpendicular transmittance (H₉₀) of thepolarizer, and determining the polarization degree through the followingexpression: polarization degree (%)={(H₀−H₉₀)/(H₀+H₉₀)}^(1/2)×100. Theparallel transmittance (H₀) described above refers to a value oftransmittance of a parallel-type laminated polarizer manufactured bycausing two identical polarizers to overlap with each other in such amanner that absorption axes thereof are parallel to each other. Inaddition, the perpendicular transmittance (H₉₀) described above refersto a value of a transmittance of a perpendicular-type laminatedpolarizer manufactured by causing two identical polarizers to overlapwith each other in such a manner that absorption axes thereof areperpendicular to each other. Note that, each transmittance is a Y valueobtained through relative spectral responsivity correction at atwo-degree field of view (C light source) in JIS Z 8701-1982.

Any appropriate polarizer may be adopted as the polarizer depending onpurpose. Examples thereof include a polarizer obtained by causing ahydrophilic polymer film such as a polyvinyl alcohol-based film, apartially formalized polyvinyl alcohol-based film, or an ethylene-vinylacetate copolymer-based partially saponified film to absorb a dichroicsubstance such as iodine or a dichroic dyestuff, followed by uniaxialstretching, and a polyene-based alignment film such as a productobtained by subjecting polyvinyl alcohol to dehydration treatment or aproduct obtained by subjecting polyvinyl chloride to dehydrochlorinationtreatment. In addition, there may also be used, for example,guest-host-type E-type and O-type polarizers each including a dichroicsubstance and a crystalline compound in which the crystallinecomposition is aligned in a fixed direction as disclosed in, forexample, U.S. Pat. No. 5,523,863, and E-type and O-type polarizers inwhich the lyotropic liquid crystals are aligned in a fixed direction asdisclosed in, for example, U.S. Pat. No. 6,049,428.

Of such polarizers, a polarizer formed of a polyvinyl alcohol-based filmcontaining iodine is suitably used from the viewpoint of having a highpolarization degree. The polyvinyl alcohol or a derivative thereof isused as a material for the polyvinyl alcohol-based film to be appliedonto the polarizer. Examples of the derivative of polyvinyl alcoholinclude polyvinyl formal and polyvinyl acetal as well as polyvinylalcohol modified with, for example, an olefin such as ethylene orpropylene, an unsaturated carboxylic acid such as acrylic acid,methacrylic acid, or crotonic acid, alkyl ester thereof, or acrylamide.Polyvinyl alcohol having a polymerization degree of about from 1,000 to10,000 and a saponification degree of about from 80 mol % to 100 mol %are generally used.

The polyvinyl alcohol-based film (unstretched film) is subjected to atleast uniaxial stretching treatment and iodine dyeing treatmentaccording to usual methods, and may further be subjected to boric acidtreatment or iodine ion treatment. In addition, the polyvinylalcohol-based film (stretched film) subjected to the treatment describedabove becomes a polarizer through drying according to a usual method.

The stretching method in the uniaxial stretching treatment is notparticularly limited, and any one of a wet stretching method and a drystretching method may be adopted. As a stretching means for the drystretching method, there is given, for example, a roll stretchingmethod, a heating roll stretching method, or a compression stretchingmethod. The stretching may be performed in a plurality of steps. In thestretching means, the unstretched film is generally in a heated state. Afilm having a thickness of about from 30 μm to 150 μm is generally usedas the unstretched film. The stretching ratio of the stretched film maybe appropriately set depending on purpose. However, the stretching ratio(total stretching ratio) is about from 2 times to 8 times, preferablyfrom 3 times to 6.5 times, more preferably from 3.5 times to 6 times.The thickness of the stretched film is suitably about from 5 μm to 40μm.

The iodine dyeing treatment is performed by immersing the polyvinylalcohol-based film in an iodine solution containing iodine and potassiumiodide. The iodine solution is generally an iodine aqueous solution, andcontains potassium iodide as iodine and a dissolution aid. Theconcentration of iodine is preferably about from 0.01 wt % to 1 wt %,more preferably from 0.02 wt % to 0.5 wt %, and the concentration ofpotassium iodide is preferably about from 0.01 wt % to 10 wt %, morepreferably from 0.02 wt % to 8 wt %.

In iodine dyeing treatment, the temperature of the iodine solution isgenerally about from 20° C. to 50° C., and is preferably from 25° C. to40° C. Time period of the immersion falls within a range of generallyabout from 10 seconds to 300 seconds, and is preferably from 20 secondsto 240 seconds. In iodine dyeing treatment, through adjustment ofconditions such as the concentration of the iodine solution, and theimmersion temperature and time period of the immersion of polyvinylalcohol-based film into the iodine solution, iodine content andpotassium content in the polyvinyl alcohol-based film is adjusted so asto allow both to fall within a desired range. The iodine dyeingtreatment may be performed at any one of the time points before theuniaxial stretching treatment, during the uniaxial stretching treatment,and after the uniaxial stretching treatment.

The boric acid treatment is performed by immersing the polyvinylalcohol-based film in a boric acid aqueous solution. The concentrationof boric acid in the boric acid aqueous solution is about from 2 wt % to15 wt %, preferably from 3 wt % to 10 wt %. With potassium iodide,potassium ion and iodine ion may be incorporated in the boric acidaqueous solution. The concentration of potassium iodide in the boricacid aqueous solution is about from 0.5 wt % to 10 wt %, and ispreferably from 1 wt % to 8 wt %. A polarizer with low coloration, thatis, almost constant absorbance over approximately entire wavelengthregion of visible light, so-called neutral grey can be obtained with aboric acid aqueous solution containing potassium iodide.

For example, an aqueous solution obtained by incorporating iodine ionwith, for example, potassium iodide is used for the iodine iontreatment. The concentration of potassium iodide is preferably aboutfrom 0.5 wt % to 10 wt %, more preferably from 1 wt % to 8 wt %. Iniodine ion immersion treatment, the temperature of the aqueous solutionis generally about from 15° C. to 60° C., and is preferably from 25° C.to 40° C. Time period of the immersion is generally about from 1 secondto 120 seconds, and preferably falls within a range of from 3 seconds to90 seconds. The time point of the iodine ion treatment is notparticularly limited as long as the time point is before the dryingstep. The treatment may be performed after water washing describedlater.

The polyvinyl alcohol-based film (stretched film) subjected to thetreatment described above may be subjected to a water washing step and adrying step according to a usual method.

Any appropriate drying method such as natural drying, drying by blowing,or drying by heating may be adopted as the drying step. In the case ofthe drying by heating, for example, drying temperature thereof istypically from 20° C. to 80° C., and is preferably from 25° C. to 70° C.Time period of the drying is preferably about from 1 minute to 10minutes. In addition, the moisture content of the polarizer after thedrying is preferably from 10 wt % to 30 wt %, more preferably from 12 wt% to 28 wt %, still more preferably from 16 wt % to 25 wt %. When themoisture content is excessively high, in drying the polarizing plate,the polarization degree is liable to decrease in accordance with thedrying of the polarizer. In particular, the perpendicular transmittancein a short wavelength region of 500 nm or less is increased, that is,the black display is liable to be colored with blue because of theleakage of the short wavelength light. On the contrary, when themoisture content of the polarizer is excessively small, a problem suchas local unevenness defect (knick defect) may easily occur.

(2) Each of the unit prisms 33 of the prism sheet 30 is not limited tothe form in which, on the cross section parallel to the array directionand parallel to the thickness direction, the cross-sectional shapethereof is asymmetrical with respect to the straight line passingthrough the vertex and perpendicular to the sheet surface, and may adopta form in which the above-mentioned cross-sectional shape is symmetricallike an isosceles triangular shape. Ina case of adopting a unit prism inwhich a cross-sectional shape is an isosceles triangular shape, it ispreferred that the brightness distribution of the light output from thelight guide plate 21 be set as a narrower distribution than in the prismsheet 30 described in the embodiment from a viewpoint of enhancing thelight condensing property. Moreover, as illustrated in FIG. 12, the unitprism 33 may adopt a polygonal shape in which a cross-sectional shape issymmetrical with respect to the straight line passing through the vertexand perpendicular to the sheet surface. A prism sheet including the unitprism 33 with such a shape in which the cross-sectional shape issymmetrical can also be applied to the dual lamp-type surface lightsource device.

A description is briefly made of the modification of the unit prism 33,which is illustrated in FIG. 12. In a unit prism 33C, both of a firstinclined surface 34C and a second inclined surface 35C each have aplurality of flat surfaces, and a cross-sectional shape of the unit prim33C is a symmetrical shape with respect to a line passing through avertex t thereof and perpendicular to the sheet surface. The unit prism33C has an approximately triangular column shape (polygonal shape)having the first inclined surface 34C including two flat surfaces 34 aand 34 b different in inclination angle, and the second inclined surface35C including two flat surfaces 35 a and 35 b different in inclinationangle. At this time, the unit prism 33C is arranged so that the firstinclined surface 34C can be located on the side surface 21 a side, andthat the second inclined surface 35C can be located on the side surface21 b side. Light entering from the side surfaces 21 a and 21 b is guidedin the light guide plate 21, and is output as the first directivitylight from the light guide plate 21 toward the unit prism 33Cillustrated in FIG. 12. This first directivity light enters from theflat surfaces 34 a and 34 b of the first inclined surface 34C and theflat surfaces 35 a and 35 b of the second inclined surface 35C. In theunit prism 33C, as described above, the inclination angles of therespective flat surfaces 34 a and 34 b of the first inclined surface 34Care angles at which the first directivity light from the light guideplate 21 is capable of entering, and are also angles at which the lightentering from the second inclined surface 35C can be reflected as thesecond directivity light having the maximum intensity in the normaldirection of the sheet surface. Moreover, the inclination angles of therespective flat surfaces 35 a and 35 b of the second inclined surface35C are angles at which the light entering from the first inclinedsurface 34C can be reflected as the second directivity light having themaximum intensity in the normal direction of the sheet surface, and areangles at which the first directivity light from the light guide plate21 is capable of entering. Preferred conditions of the inclinationangles of the respective flat surfaces 34 a and 34 b of the firstinclined surface 34C are similar to the above-mentioned preferredconditions on the respective flat surfaces of the second inclinedsurface 35 illustrated and shown in FIGS. 6( a) and 6(b) and FIG. 7( a).Such a form is adopted for the unit prism 33 so that, also in the liquidcrystal display apparatus including the dual lamp-type surface lightsource device, the utilization efficiency of the light can be enhanced,and a bright image can be displayed. Note that, without being limited tothe shape as described above, the unit prism 33 may be a trapezoid inwhich a vertex portion of a triangle is changed to a short upper side,or may have a curved shape in which at least one of the inclinedsurfaces protrudes to the light guide plate 21 side.

(3) The light guide plate 21 is not limited to the form in which thethickness of the base portion 22 is approximately constant. In the caseof providing the light source unit 10 on one side surface side, thelight guide plate 21 may have a tapered shape, which is thickest on theside surface 21 a side on which the light source unit 10 is provided,and becomes gradually thinner as being closer to the opposed sidesurface 21 b side. By adopting such a form, the utilization efficiencyof the light and the evenness in brightness can be enhanced. Moreover,in the case of the dual lamp-type surface light source device in whichthe light source unit 10 is arranged on both the side surfaces 21 a and21 b of the light guide plate 21, the light guide plate 21 may be alight guide plate in which the back surface side is formed into an archshape having a thin center portion. Further, the light guide plate 21may have a form including the back surface-side unit optical elements 26and the light output-side unit optical elements 24, which are describedin JP 2007-220347 A, JP 2011-90832 A, JP 2004-213019 A, JP 2008-262906A, and the like.

(4) The prism sheet 30 may adopt a form including a light diffusionlayer in order to impart a light diffusion function as needed to anextent of not disturbing the polarization. For example, as the lightdiffusion layer, a layer having light-diffusing microparticles dispersedin a light-transmissive resin, and the like can be used. This lightdiffusion layer can be provided at any position of the prism sheet 30.For example, the light diffusion layer may be provided on the lightoutput surface 30 a, or may be provided between the base portion 31 andthe prism portion 32 in the thickness direction.

(5) The light output surface 30 a of the prism sheet 30 is not limitedto the form of the flat and smooth surface, and for example, on thefront surface of the light output surface 30 a, there may be formed amat layer having a group of micro protrusions in which an averageprotrusion height (for example, ten point average roughness Rzprescribed in JIS B 0601 (1994 version)) is preferably the visible lightwavelength range (typically 0.38 μm to 0.78 μm) or more. By forming themat layer, interference with another member due to intimate contacttherewith can be prevented, and/or an appearance defect such as a flawcan be hidden. It is preferred that the mat layer have a micro unevenshape to an extent of not disturbing the polarization direction of thefirst directivity light. The mat layer can be formed by appropriatelyusing mat agent coating, embossing, or the like. In a case of coating amat agent, a stretch balance between the prism portion 32 and the matlayer formed by the coating can be adjusted, and an effect is obtainedthat deformation of the prism sheet 30, such as warp and deflection, issuppressed.

(6) Depending on the purpose, the surface light source device 20 mayfurther include any appropriate optical sheet at any appropriateposition. For example, the surface light source device 20 may include alight diffusion sheet, a lens array sheet, or the like between the lightguide plate 21 and the prism sheet 30 and/or between the prism sheet 30and the liquid crystal display panel 15. In a case where the surfacelight source device includes the light diffusion sheet, a viewing angleof the liquid crystal display apparatus can be widened.

(7) Depending on the purpose, the liquid crystal display apparatus mayfurther include any appropriate optical compensation film (in thisspecification, also referred to as an anisotropic optical element, aretardation film, and a compensation plate in some cases) at anyappropriate position. An arrangement position of the opticalcompensation film, the number of films for use, birefringence (indexellipsoid) thereof, and the like can be selected appropriately dependingon a drive mode of the crystal cell, desired characteristics, and thelike.

For example, when the liquid crystal cell employs the IPS mode, theliquid crystal display apparatus may include a first anisotropic opticalelement that satisfies a relationship of nx₁>ny₁>nz₁ and is arrangedbetween the liquid crystal cell 12 and the second polarizing plate 14,and a second anisotropic optical element that satisfies a relationshipof nz₂>nx₂>ny₂ and is arranged between the first anisotropic opticalelement and the liquid crystal cell. The second anisotropic opticalelement may be a so-called positive C plate that satisfies arelationship of nz₂>nx₂=ny₂. The slow axis of the first anisotropicoptical element and the slow axis of the second anisotropic opticalelement may be perpendicular or parallel, and it is preferred that theslow axes be parallel in consideration of the viewing angle andproductivity. Further, a preferred range of each retardation in thiscase is as follows.

-   60 nm<Re₁<140 nm-   1.1<Nz₁<1.7-   10 nm<Re₂<70 nm-   −120 nm<Rth₂<−40 nm    In the expressions, Re represents the in-plane retardation of the    anisotropic optical element as defined above. Rth represents the    thickness direction retardation of the anisotropic optical element,    and is represented by Rth={(nx₁+n_(Y2))/2−nz₂}×d₂. Nz represents an    Nz coefficient, and is represented by Nz=(nx₁−nz₁)/(nx₁−ny₁). In the    expressions, nx and ny are as defined above. nz represents a    thickness direction refractive index of the optical member (in this    case, the first anisotropic optical element or the second    anisotropic optical element). Note that, subscripts “1” and “2”    represent the first anisotropic optical element and the second    anisotropic optical element, respectively.

Alternatively, the first anisotropic optical element may satisfy arelationship of nx₁>nz₁>ny₁, and the second anisotropic optical elementmay be a so-called negative C plate, which satisfies a relationship ofnx₂=ny₂>nz₂. Note that, herein, for example, “nx=ny” encompasses notonly a case where nx and ny are strictly equal to each other but also acase where nx and ny are substantially equal to each other. Thedescription: “substantially equal” used herein means to encompass a casewhere nx and ny differ from each other in such a range that overalloptical characteristics of the liquid crystal display apparatus are notpractically affected. Therefore, the negative C plate in this embodimentencompasses the case where the plate has biaxiality.

The second anisotropic optical element may be omitted depending on thepurpose or desired characteristics.

When the liquid crystal cell employs the IPS mode, the liquid crystaldisplay panel may be in a so-called O mode or a so-called E mode. Thedescription: “liquid crystal display panel in the O mode” refers to apanel in which the absorption axis direction of the polarizer arrangedon the light source side of the liquid crystal cell is substantiallyparallel to the initial alignment direction of the liquid crystal cell.The description: “liquid crystal panel in the E mode” refers to a panelin which the absorption axis direction of the polarizer arranged on thelight source side of the liquid crystal cell is substantiallyperpendicular to the initial alignment direction of the liquid crystalcell. The description: “initial alignment direction of the liquidcrystal cell” refers to a direction in which in absence of the electricfield, the in-plane refractive index of the liquid crystal layerobtained as a result of alignment of liquid crystal molecules containedin the liquid crystal layer becomes maximum. In the case of the O mode,the above-mentioned anisotropic optical element may be arranged betweenthe first polarizing plate and the liquid crystal cell, and in the caseof the E mode, the above-mentioned anisotropic optical element may bearranged between the second polarizing plate and the liquid crystalcell.

In addition, for example, when the liquid crystal cell employs the VAmode, in the liquid crystal display apparatus, a circularly polarizingplate may be used as the polarizing plate. That is, the first polarizingplate may include an anisotropic optical element that functions as a λ/4plate on the liquid crystal cell side of the polarizer, and the secondpolarizing plate may include an anisotropic optical element thatfunctions as a λ/4 plate on the liquid crystal cell side of thepolarizer. The second polarizing plate may include another anisotropicoptical element with a relationship of refractive indices of nz>nx>nybetween the above-mentioned anisotropic optical element and thepolarizer. Further, when acell represents a retardation wavelengthdispersion value (Recell[450]/Recell[550]) of the liquid crystal cell,and α(λ/4) represents an average retardation wavelength dispersion value(Re(λ/4)[450]/Re(λ/4)[550]) of the anisotropic optical elements of theabove-mentioned first polarizing plate and the above-mentioned secondpolarizing plate, α(λ/4)/a cell is preferably 0.95 to 1.02. Further, anangle formed between the absorption axis of the polarizer of the firstpolarizing plate and the slow axis of the above-mentioned anisotropicoptical element is preferably substantially 45° or substantially 135°.In addition, it is preferred that the Nz coefficient of theabove-mentioned anisotropic optical element satisfy a relationship of1.1<Nz≦2.4, and that the Nz coefficient of the above-mentioned anotheranisotropic optical element satisfy a relationship of −2≦Nz≦−0.1.

When the liquid crystal cell employs the VA mode, in the liquid crystaldisplay apparatus, a linearly polarizing plate may also be used as thepolarizing plate. That is, the first polarizing plate may include ananisotropic optical element other than the λ/4 plate on the liquidcrystal cell side of the polarizer, and the second polarizing plate mayinclude an anisotropic optical element other than the λ/4 plate on theliquid crystal cell side of the polarizer. Each of the anisotropicoptical elements of the above-mentioned first polarizing plate and theabove-mentioned second polarizing plate may be one element or two ormore elements. Such an anisotropic optical element in the linearlypolarizing plate compensates, through birefringence, light leakagecaused by birefringence of the liquid crystal cell, shift of an apparentangle of the absorption axis of the polarizer in the case of viewingfrom an oblique direction, or the like. Depending on the purpose or thelike, any appropriate optical characteristics may be used as the opticalcharacteristics thereof. For example, it may be preferred that theabove-mentioned anisotropic optical element satisfy a relationship ofnx>ny>nz. More specifically, the in-plane retardation Re of theanisotropic optical element is preferably from 20 nm to 200 nm, morepreferably from 30 nm to 150 nm, still more preferably from 40 nm to 100nm. The thickness direction retardation Rth of the anisotropic opticalelement is preferably from 100 nm to 800 nm, more preferably from 100 nmto 500 nm, still more preferably from 150 nm to 300 nm. The Nzcoefficient of the anisotropic optical element is preferably from 1.3 to8.0.

(8) The light guide plate 21 may contain a light scattering material.For example, the base portion 22 of the light guide plate 21 may containan approximately uniformly dispersed light scattering material(light-diffusing particle: not shown). The light scattering material hasa function of changing a traveling direction of light traveling in thebase portion 22 through, for example, reflection or refraction todiffuse (scatter) the light. As the light scattering material, there maybe used a particle formed of a material having a refractive indexdifferent from that of the base of the base portion 22 or a particleformed of a material having a reflection action for light. The materialproperty, the average particle diameter, the refractive index, and thelike of the light scattering material may appropriately be adjusteddepending on intensity of directivity required for outputting light fromthe light guide plate 21. In the material property, the average particlediameter, the refractive index, and the like of the light scatteringmaterial, for example, the ranges disclosed in JP 3874222 B2 may beadopted. The entire disclosure of JP 3874222 B2 is incorporated byreference herein. As a material for forming the light scatteringmaterial, there is given, for example, a particle made of a transparentsubstance such as silica (silicon dioxide), alumina (aluminum oxide), anacrylic resin, a PC resin, and a silicone-based resin. In this form, itis preferred to provide the back surface-side unit optical element 26 asillustrated in FIG. 1, FIGS. 4( a) and 4(b), and FIGS. 5( a) and 5(b).

(9) Note that, in a general liquid crystal display apparatus, the firstpolarizing plate is generally arranged in such a manner that itspolarized light component in the vertical direction is transmitted andits polarized light component in the horizontal direction is absorbed inconsideration of a case where the liquid crystal display apparatus isviewed while wearing polarizing sunglasses. However, in the presentinvention, when the first polarizing plate and the second polarizingplate are arranged so as to utilize the polarized light component of thelight source device, the transmission axis of the first polarizing plateis approximately perpendicular to the transmission axis of thepolarizing sunglasses in some cases. Therefore, in the presentinvention, there may be used an optical member for partially or entirelychanging or eliminating the polarization state or the polarization axisangle on the viewer side of the first polarizing plate (such as a λ/4plate, a λ/2 plate, a high retardation film, or a scattering element).

(10) As described above, one of the features of the present invention isthat the second directivity light contains a large amount of Pcomponents of polarized light so as to match with the transmission axisof the second polarizing plate, thereby improving the light utilizationefficiency. That is, the present invention realizes improvement of thelight utilization efficiency by arranging the liquid crystal displaypanel in such a manner that the YZ plane of the light guide plate isparallel to the transmission axis of the second polarizing plate andthus the absorption axis of the second polarizing plate is perpendicularto the YZ plane. However, as described above, depending on the azimuthangle of the first polarizing plate, a problem may arise as in the caseof using the polarizing sunglasses. Therefore, a λ/2 plate may be usedin order to freely set the angle of the absorption axis of thepolarizing plate used for the liquid crystal display panel.Specifically, the λ/2 plate is arranged between the second polarizingplate and the prism portion, and thus, the polarization direction canoptimally be changed for use. In this case, the λ/2 plate may bearranged between the polarized light selective reflection sheet and theprism portion, or may be arranged between the polarized light selectivereflection sheet and the second polarizing plate. When the λ/2 plate isarranged between the polarized light selective reflection sheet and theprism portion, the λ/2 plate may be arranged in such a manner that theslow axis of the λ/2 plate is in the direction between the direction ofthe transmission axis of the polarized light selective reflection sheetand the direction of the YZ plane of the light guide plate. In thiscase, it may be preferred that the λ/2 plate be arranged in such amanner that its slow axis is in an average angle of the angle(direction) of the transmission axis of the polarized light selectivereflection sheet and the angle (direction) of the YZ plane of the lightguide plate. When the λ/2 plate is arranged between the polarized lightselective reflection sheet and the second polarizing plate, thetransmission axis of the polarized light selective reflection sheet maybe arranged in parallel to the YZ plane, and the slow axis of the λ/2plate may be arranged in the direction between the direction of thetransmission axis of the second polarizing plate and the direction ofthe transmission axis of the polarized light selective reflection sheet.In this case, it may be preferred that the λ/2 plate be arranged in sucha manner that its slow axis is in the average axis of the angle(direction) of the transmission axis of the second polarizing plate andthe angle (direction) of the transmission axis of the polarized lightselective reflection sheet.

EXAMPLES

The present invention is specifically described below by way ofexamples, but the present invention is not limited to these examples.Testing and evaluating methods in the examples are as follows. Moreover,unless particularly specified, “parts” and “%” in the examples areweight-based units.

(1) Brightness of Liquid Crystal Display Apparatus

Brightness of each of liquid crystal display apparatuses obtained in theexamples and comparative examples was evaluated in accordance with JIS C8152 “Photometry of white light emitting diode for general lighting”.Specifically, a total luminous flux of light beams output from theliquid crystal display apparatus was measured by an integrating sphere(trade name: CSTM-LMS-760 Type) manufactured by Labsphere, Inc. in theU.S.A., and an accumulated luminous flux sum was obtained. By takingComparative Example 1 as a reference (100%), an accumulated luminousflux ratio was taken as an evaluation criterion of the brightness.

(2) Direction of Maximum Intensity of First Directivity Light L1

In each of the surface light source devices obtained in the examples andthe comparative examples, a direction of the maximum intensity of thefirst directivity light L1 output from the light guide plate illustratedin FIGS. 6( a) and 6(b) was measured by a luminance meter (trade name:BM-7) manufactured by Topcon Corporation and by a goniometer, and wasobtained as the angle θ from the normal when the normal was defined as0°. In the measurement, the center portion of the light guide plate wasset at a two-degree field of view of the luminance meter, and thesurface light source device was inclined in the Y2-axis direction fromthe Z2-axis, which were illustrated in FIG. 1, by using the goniometer.In this manner, the measurement was performed.

(3) Ratio of Polarized Light Component Oscillating in PlaneApproximately Parallel to Light Guiding Direction of Light

In each of the surface light source devices obtained in the examples andthe comparative examples, a ratio of the polarized light component ofthe second directivity light L2 output from the prism sheet illustratedin FIGS. 6( a) and 6(b) was measured by the luminance meter (trade name:BM-7) manufactured by Topcon Corporation and by the goniometer, and wasobtained by the following expression when the arrow D1 was set at 0°bymatching the transmission axis of the polarizing plate with the arrowD1 direction of FIGS. 11( a) to 11(d).Polarization ratio=0° brightness value/(0° brightness value+90°brightness value)×100%

In the measurement, the center portion of the light guide plate was setat a two-degree field of view of the luminance meter, and the surfacelight source device was rotated by 0° and 90° in the X-Y plane directionof FIGS. 11( a) to 11(d) by the goniometer. In this manner, themeasurement was performed.

(4) Output Characteristics from Surface Light Source

Output characteristics of each of the light guide plates were indicatedby the half width angle of the light output in a parallel direction tothe arrangement of the light sources of the surface light source. As ameasurement method, for each of the surface light source devicesobtained in the examples and the comparative examples, a distribution ofoutput from the center portion of the surface light source was measuredby using the above-mentioned EZ contrast, and the output characteristicswere indicated as an angular width in the parallel direction to thearrangement of the light sources of the surface light source. Theangular width shows brightness with a ½ value of that of the peakbrightness.

Example 1

(A) Manufacture of Light Guide Plate

By using an acrylic resin containing a light scattering material, lightoutput-side unit optical elements and back surface-side unit opticalelements were shaped on a sheet serving as the base portion so that alight guide plate as illustrated in FIG. 1 and FIGS. 4( a) and 4(b) wasmanufactured. Here, unlike FIG. 4( a), the back surface-side unitoptical elements had a shape adapted to the single lamp-type surfacelight source device (wedge-like prism columnar shape in which across-sectional shape was an asymmetrical shape on the cross sectionparallel to the array direction and parallel to the thicknessdirection). A ridge line direction of the back surface-side unit opticalelements was set parallel to the array direction (X direction) of thepoint light sources of the light source unit. As illustrated in FIG. 13,each of the light output-side unit optical elements had a shape similarto an isosceles triangular column shape (prism shape with a pentagonalcross section in which base angles were θ1=θ2=45° and a portion of 50%in a prism tip end portion when a pitch was defined to be 100% wasformed into a prism with a vertex angle of 140°), and a ridge linedirection thereof was set as a direction (Y direction) perpendicular tothe ridge line direction of the back surface-side unit optical elements.This light guide plate output the first directivity light L1 having themaximum intensity in a direction of approximately 73° to the sidesurface side (opposite to the light source unit side) with respect tothe normal direction of the light output surface in the YZ plane. Thislight guide plate is hereinafter referred to as a “double-sided prismA”in some cases for convenience.

(B) Reflection Sheet

As the reflection sheet, a silver reflection sheet was used, in whichsilver was deposited on a surface of a base (PET sheet).

(C) Manufacture of Prism Sheet

As the base portion, a triacetyl cellulose (TAC) film (product name:“Fujitac ZRF80S” manufactured by Fujifilm Corporation; thickness: 80 μm)was used. A predetermined die on which the TAC film was arranged wasfilled with an ultraviolet-curable urethane acrylate resin as a prismmaterial, and ultraviolet rays were radiated thereonto, to thereby curethe prism material. In this manner, a prism sheet as illustrated in FIG.8 was manufactured. The in-plane retardation Re of the base portion was0 nm, and the thickness direction retardation Rth was 5 nm. Each of unitprisms was a triangular column prism, in which a cross-sectional shapeparallel to the array direction and parallel to the thickness directionwas a scalene triangular shape, and a first inclined surface on thelight source unit side was a steeper inclined surface (φ1<φ2) than asecond inclined surface on the other side (refer to FIG. 8). A ridgeline direction of the unit prism was set parallel to the array direction(X direction) of the point light sources of the light source unit.

(D) Point Light Source

As the point light source, an LED light source was used, and a pluralityof LED light sources were arrayed, to thereby form the light sourceunit.

(E) Manufacture of Surface Light Source Device

The above-mentioned light guide plate, reflection sheet, prism sheet,and point light source were assembled to one another in the arrangementas illustrated in FIG. 1 so that a surface light source device wasmanufactured. Note that, all of such surface light source devices usedin this example and in Examples 2 to 8 and Comparative Examples 1 to 3,which are described below, are the single lamp-type surface light sourcedevices unlike the surface light source device illustrated in FIG. 1 andFIGS. 4( a) and 4(b).

(F) Manufacture of Polarizing Plate with Compensation Plate for IPS

(F-1) Manufacture of First Anisotropic Optical Element

A commercially available polymer film (trade name: “ZeonorFilm ZF14-130(thickness: 60 μm, glass transition temperature: 136° C.)”manufacturedby Optes Inc.) whose main component was a cyclic polyolefin-basedpolymer was subjected to fixed-end uniaxial stretching in its widthdirection with a tenter stretching machine at a temperature of 158° C.in such a manner that its film width was 3.0 times as large as theoriginal film width (lateral stretching step). The resultant film was anegative biaxial plate having a fast axis in the conveying direction.The negative biaxial plate had a front retardation of 118 nm and an Nzcoefficient of 1.16.

(F-2) Manufacture of Second Anisotropic Optical Element

A pellet-shaped resin of a styrene-maleic anhydride copolymer (productname: “DYLARK D232” manufactured by Nova Chemicals Japan Ltd.) wasextruded with a single screw extruder and a T die at 270° C., and theresultant sheet-shaped molten resin was cooled with a cooling drum toobtain a film having a thickness of 100 μm. The film was subj ected tofree-end uniaxial stretching in the conveying direction with a rollstretching machine at a temperature of 130° C. and a stretching ratio of1.5 times to obtain a retardation film having a fast axis in theconveying direction (longitudinal stretching step). The resultant filmwas subjected to fixed-end uniaxial stretching in its width directionwith a tenter stretching machine at a temperature of 135° C. in such amanner that its film width was 1.2 times as large as the film widthafter the longitudinal stretching, thereby obtaining a biaxiallystretched film having a thickness of 50 μm (lateral stretching step).The resultant film was a positive biaxial plate having a fast axis inthe conveying direction. The positive biaxial plate had a frontretardation Re of 20 nm and a thickness direction retardation Rth of −80nm.

(F-3) Manufacture of Polarizing Plate with Compensation Plate for IPS

50 parts by weight of methylolmelamine were dissolved in pure water toprepare an aqueous solution with a solid content of 3.7 wt %, and anaqueous solution containing alumina colloid having a positive charge(average particle diameter: 15 nm) at a solid content of 10 wt % withrespect to 100 parts by weight of the aqueous solution was prepared. 18parts by weight of the aqueous solution were added with respect to 100parts by weight of a polyvinyl alcohol-based resin having an acetoacetylgroup (average polymerization degree: 1,200, saponification degree:98.5%, acetoacetylation degree: 5 mol %) to prepare an aluminacolloid-containing adhesive. The resultant alumina colloid-containingadhesive was applied onto one surface of the triacetyl cellulose (TAC)film (product name: “KC4UW” manufactured by Konica Minolta, Inc.;thickness: 40 μm). On the other hand, a polymer film (trade name “9P75R(thickness: 75 μm, average polymerization degree: 2,400, saponificationdegree: 99.9%)” manufactured by KURARAY CO., LTD.) whose main componentwas polyvinyl alcohol was stretched 1.2-fold in the conveying directionwhile being immersed in a water bath for 1 minute, was stretched 3-foldwith reference to the non-stretched film (original length) in theconveying direction while being dyed through immersion in an aqueoussolution with a concentration of iodine of 0.3 wt % for 1 minute, wasstretched 6-fold with reference to the original length in the conveyingdirection while being immersed in an aqueous solution with aconcentration of boric acid of 4 wt % and a concentration of potassiumiodide of 5 wt %, and was dried at 70° C. for 2 minutes to produce apolarizer. The above-mentioned TAC film/alumina colloid-containingadhesive laminate was laminated onto one surface of the resultantpolarizer by roll-to-roll in such a manner that the conveying directionsthereof were parallel to each other. Subsequently, the first anisotropicoptical element having a surface to which the alumina colloid-containingadhesive had been applied was laminated onto the opposite surface of thepolarizer by roll-to-roll in such a manner that the conveying directionsthereof were parallel to each other. After that, the resultant was driedat 55° C. for 6 minutes to obtain a polarizing plate having a singleaxis transmittance of 43.2% at a wavelength of 589 nm (first opticalanisotropic element/polarizer/TAC film). The second optical anisotropicelement was laminated onto a surface of the first optical anisotropicelement of the polarizing plate through intermediation of an acrylicpressure-sensitive adhesive (thickness: 5 μm) by roll-to-roll in such amanner that the conveying directions thereof were parallel to eachother, thereby obtaining a polarizing plate with a compensation platefor IPS.

(G) Manufacture of Liquid Crystal Display Apparatus

A liquid crystal display panel was taken out of a liquid crystal displayapparatus of the IPS mode (trade name: “iPad2” manufactured by AppleInc.), and an optical member such as a polarizing plate was removed fromthe liquid crystal display panel to take out a liquid crystal cell. Bothsurfaces (outside of each glass substrate) of the liquid crystal cellwere cleaned for use. As the first polarizing plate, a commerciallyavailable polarizing plate (product name: “CVT1764FCUHC” manufactured byNitto Denko Corporation) was attached onto the upper side of the liquidcrystal cell (viewer side). Further, in order to improve visibility inviewing the liquid crystal display apparatus while wearing polarizingsunglasses, a λ/4 wavelength plate (trade name: “UTZ film #140”manufactured by Kaneka Corporation) was attached onto the firstpolarizing plate through intermediation of an acrylic pressure-sensitiveadhesive in such a manner that its slow axis formed an angle of 45° withrespect to the absorption axis of the first polarizing plate. Inaddition, as the second polarizing plate, the polarizing plate with acompensation plate for IPS, which had been obtained in the section (F),was attached onto the lower side of the liquid crystal cell (lightsource side) through intermediation of an acrylic pressure-sensitiveadhesive to obtain a liquid crystal display panel. In this case, thepolarizing plates were attached in such a manner that the transmissionaxis of the first polarizing plate was in the X direction in FIG. 1 andthe transmission axis of the second polarizing plate was in the Ydirection in FIG. 1. The surface light source device manufactured in thesection (E) was assembled to the liquid crystal display panel tomanufacture a liquid crystal display apparatus illustrated in FIG. 1.The resultant liquid crystal display apparatus was subjected to theabove-mentioned evaluations (1) to (4). Table 1 shows the results.

Example 2

A liquid crystal display apparatus was manufactured in a similar mannerto Example 1 except that a polarized light selective reflection sheet(product name: “DBEF” manufactured by 3M Company) was arranged betweenthe second polarizing plate of the liquid crystal display panel and theprism sheet. The obtained liquid crystal display apparatus was subjectedto the above-mentioned evaluations (1) to (4). Table 1 shows theresults.

Comparative Example 1

A light guide plate, in which a white PET sheet was used as a reflectionsheet, and a dot-like light diffusion layer was formed on a back surfaceside, was used. This light guide plate did not include the backsurface-side unit optical elements or the light output-side unit opticalelements, and a light scattering layer of the light guide plate had agray-scale pattern in which a size of dots became larger as being awayfrom the light source unit. In this light guide plate, the direction(first direction; output angle: α) in which the first directivity lightTA had the maximum intensity was located in the vicinity of 65°, butsuch light in which an output angle was distributed more widely than inthe light guide plate used in the examples was output therefrom.Moreover, the white PET sheet was used as the reflection sheet. A liquidcrystal display apparatus was manufactured in a similar manner toExample 1 except that the light guide plate and the reflection sheet,which were as described above, were used, that arrangement was made soas to set the transmission axis of the first polarizing plate of theliquid crystal display panel along the Y direction and to set thetransmission axis of the second polarizing plate along the X direction,and that the polarized light selective reflection sheet was arrangedbetween the second polarizing plate of the liquid crystal display paneland the prism sheet. The obtained liquid crystal display apparatus wassubjected to the above-mentioned evaluations (1) to (4). Table 1 showsthe results.

Comparative Example 2

A liquid crystal display apparatus was manufactured in a similar mannerto Example 1 except that the arrangement was made so as to set thetransmission axis of the first polarizing plate of the liquid crystaldisplay panel along the Y direction and to set the transmission axis ofthe second polarizing plate along the X direction. The obtained liquidcrystal display apparatus was subjected to the above-mentionedevaluations (1) to (4). Table 1 shows the results.

Comparative Example 3

A liquid crystal display apparatus was manufactured in a similar mannerto Comparative Example 1 except that the polarized light selectivereflection sheet was not used. The obtained liquid crystal displayapparatus was subjected to the above-mentioned evaluations (1) to (4).Table 1 shows the results.

Example 3

A light guide plate different in the cross-sectional shape of the lightoutput-side unit optical elements (this light guide plate is hereinafterreferred to as a double-sided prism. B in some cases) was manufacturedin a similar manner to the double-sided prism A of Example 1.Specifically, in the double-sided prism B, each of the light output-sideunit optical elements had a prism shape in which a cross-section had aright-angled isosceles triangular column shape (base angles: θ1=θ2=45°;vertex angle: 90°), and a ridge line direction thereof was set as thedirection (Y direction) perpendicular to the ridge line direction of theback surface-side unit optical elements. A liquid crystal displayapparatus was manufactured in a similar manner to Example 2 except thatthis double-sided prism B was used as the light guide plate in place ofthe double-sided prism A, and that an acrylic resin film (in-planeretardation Re=3 nm, thickness direction retardation Rth=10 nm,thickness=40 μm) was used as the base portion of the prism sheet inplace of the TAC film. The obtained liquid crystal display apparatus wassubjected to the above-mentioned evaluations (1) to (4). Table 1 showsthe results. Note that, this acrylic resin film was manufactured in thefollowing manner. 100 parts by weight of an imidized MS resin describedin Production Example 1 of JP 2010-284840 A and 0.62 part by weight of atriazine-based ultraviolet absorber (trade name: T-712 manufactured byAdeka Corporation) were mixedwith each other at 220° C. by a biaxialkneader, so that resin pellets were prepared. The obtained resin pelletswere dried at 100.5 kPa and 100° C. for 12 hours, were extruded from a Tdie at a die temperature of 270° C. by a uniaxial extruder, and wereformed into a film shape (thickness: 160 μm). Moreover, the film wasstretched in a conveying direction thereof under an atmosphere of 150°C. (thickness: 80 μm), and subsequently, was stretched in a directionperpendicular to the conveying direction under an atmosphere of 150° C.so that a film with a thickness of 40 μm was obtained.

Example 4

A light guide plate different in the cross-sectional shape of the lightoutput-side unit optical elements (this light guide plate is hereinafterreferred to as a double-sided prism C in some cases) was manufactured ina similar manner to the double-sided prism A of Example 1. Specifically,in the double-sided prism C, each of the light output-side unit opticalelements had a prism shape in which a cross-section had an isoscelestriangular column shape (base angles: θ1=θ2=20°; vertex angle: 140°),and a ridge line direction thereof was set as the direction (Ydirection) perpendicular to the ridge line direction of the backsurface-side unit optical elements. A liquid crystal display apparatuswas manufactured in a similar manner to Example 2 except that thisdouble-sided prism C was used as the light guide plate in place of thedouble-sided prism A, and that the acrylic resin film of Example 3 wasused as the base portion of the prism sheet in place of the TAC film.The obtained liquid crystal display apparatus was subjected to theabove-mentioned evaluations (1) to (4). Table 1 shows the results.

Example 5

A light guide plate different in the cross-sectional shape of the lightoutput-side unit optical elements (this light guide plate is hereinafterreferred to as a double-sided prism D in some cases) was manufactured ina similar manner to the double-sided prism A of Example 1. Specifically,in the double-sided prism D, each of the light output-side unit opticalelements had a prism shape in which a cross-section had a shape similarto an isosceles triangular column shape (shape inwhich a base portion ofthe isosceles triangle having base angles of θ1=θ2=20° and a vertexangle of 140° had a curved shape in cross section), and a ridge linedirection thereof was set as the direction (Y direction) perpendicularto the ridge line direction of the back surface-side unit opticalelements. A liquid crystal display apparatus was manufactured in asimilar manner to Example 2 except that this double-sided prism D wasused as the light guide plate in place of the double-sided prism A, andthat the acrylic resin film of Example 3 was used as the base portion ofthe prism sheet in place of the TAC film. The obtained liquid crystaldisplay apparatus was subjected to the above-mentioned evaluations (1)to (4). Table 1 shows the results.

Example 6

A liquid crystal display apparatus was manufactured in a similar mannerto Example 2 except that the acrylic resin film of Example 3 was used asthe base portion of the prism sheet in place of the TAC film. Theobtained liquid crystal display apparatus was subjected to theabove-mentioned evaluations (1) to (4). Table 1 shows the results.

Example 7

A liquid crystal display apparatus was manufactured in a similar mannerto Example 6 except that a liquid crystal display panel was taken out ofa liquid crystal display apparatus of an MVA mode (trade name:“KDL20J3000” manufactured by Sony Corporation) in place of the liquidcrystal display apparatus of the IPS mode, and that a liquid crystalcell of this panel was used. The obtained liquid crystal displayapparatus was subjected to the above-mentioned evaluations (1) to (4).Table 1 shows the results.

Example 8

A liquid crystal display apparatus was manufactured in a similar mannerto Example 2 except that the dual lamp-type surface light source deviceas illustrated in FIG. 1 and FIGS. 4( a) and 4(b) was used, that, incorrespondence thereto, a double-sided prism E being a dual lamp-typedesign of the double-sided prism A (in which a cross-sectional shape ofthe back surface-side unit optical elements was the wedge-like prismcolumnar shape having the asymmetrical shape on the cross sectionparallel to the array direction and parallel to the thickness directionas illustrated in FIG. 6( b)) was used as the light guide plate, andthat the acrylic resin film of Example 3 was used as the base portion ofthe prism sheet in place of the TAC film. The obtained liquid crystaldisplay apparatus was subjected to the above-mentioned evaluations (1)to (4). Table 1 shows the results.

TABLE 1 Polarized Direction of light Prism maximum Ratio of Output UpperLiquid Lower selective sheet Light Accumulated intensity in polarizedcharacteristics polarizing crystal polarizing reflection base guideReflection luminous light guide light of surface light plate panel platesheet portion plate sheet flux ratio plate component source Example 1 0°IPS 90° None TAC Double- Silver  86%  76° 55% 47° sided   prism A  Example 2 0° IPS 90° Present TAC Double- Silver 131%  76° 55% 47° sided  prism A   Comparative 90° IPS 0° Present TAC Dot White PET 100%  75°51% 53° Example 1 pattern   Comparative 90° IPS 0° None TAC Double-Silver  73%  76° 55% 47° Example 2 sided   prism A   Comparative 90° IPS0° None TAC Dot White PET  67%  75° 51% 53° Example 3 pattern   Example3 0° IPS 90° Present Acrylic Double- Silver 135%  76° 55% 42° resinsided   prism B   Example 4 0° IPS 90° Present Acrylic Double- Silver115%  76° 54% 49° resin sided   prism C   Example 5 0° IPS 90° PresentAcrylic Double- Silver 110%  76° 53% 52° resin sided   prism D   Example6 0° IPS 90° Present Acrylic Double- Silver 130%  76° 55% 47° resinsided   prism A   Example 7 0° MVA 90° Present Acrylic Double- Silver110%  76° 55% 47° resin sided   prism A   Example 8 0° IPS 90° PresentAcrylic Double- Silver 133% ±76° 55% 47° resin sided prism E

<Evaluation>

The polarized light selective reflection sheet is extremely expensive.Accordingly, when the polarized light selective reflection sheet can beomitted in the liquid crystal display apparatus, great advantages arebrought from all viewpoints of the cost, the manufacturing efficiency,and the thinning. Meanwhile, in order to maintain the favorable displayquality, it is necessary to obtain a total luminous flux of 80% or morewith respect to the case of using the polarized light selectivereflection sheet. Here, as is apparent from Table 1, in the liquidcrystal display apparatus of Example 1, the accumulated luminous fluxratio of the total luminous flux was 80% or more of that of the liquidcrystal display apparatus including the polarized light selectivereflection sheet, and a bright and favorable image was able to bedisplayed even when the polarized light selective reflection sheet wasnot provided. Moreover, in the liquid crystal display apparatus ofExample 2, which included the polarized light selective reflectionsheet, the accumulated luminous flux ratio thereof with respect to thecorresponding liquid crystal display apparatus (Comparative Example 1)with the conventional configuration became 134%, and the liquid crystaldisplay apparatus of Example 2 became a display apparatus, which hadsignificantly higher brightness and higher utilization efficiency of thelight.

As is apparent from comparison between Example 1 and Comparative Example2, when viewed from the front direction of the liquid crystal displayapparatus, the transmission axis direction of the second polarizingplate 14 and the main polarization direction of the light (seconddirectivity light L2) output from the prism sheet 30 were set parallelto each other so that the brightness was able to be enhanced byincreasing the transmitted light amount.

As described above, according to the present invention, with arelatively simple configuration, the utilization efficiency of the lightin the liquid crystal display apparatus can be enhanced, and the brightimage can be displayed. Moreover, according to the present invention,even when the expensive polarized light selective reflection sheet orthe like is not used, the utilization efficiency of the light is high,and favorable brightness can be realized.

Industrial Applicability

The liquid crystal display apparatus of the present invention can beused for various applications such as portable devices including apersonal digital assistant (PDA), a cellular phone, a watch, a digitalcamera, and a portable gaming machine, OA devices including a personalcomputer monitor, a notebook-type personal computer, and a copyingmachine, electric home appliances including a video camera, a liquidcrystal television set, and a microwave oven, on-board devices includinga reverse monitor, a monitor for a car navigation system, and a caraudio, exhibition devices including an information monitor for acommercial store, security devices including a surveillance monitor, andcaring/medical devices including a caring monitor and a medical monitor.

Reference Signs List

-   1 liquid crystal display apparatus-   10 light source unit-   10 a point light source-   11 reflection sheet-   12 liquid crystal cell-   13 first polarizing plate-   14 second polarizing plate-   15 liquid crystal display panel-   16 polarized light selective reflection sheet-   20 surface light source device-   21 light guide plate-   24 light output-side unit optical element-   26 back surface-side unit optical element-   30 prism sheet-   31 base portion-   33 unit prism-   34 first inclined surface-   35 second inclined surface

The invention claimed is:
 1. A liquid crystal display apparatus,comprising: a liquid crystal display panel comprising a liquid crystalcell between a first polarizing plate provided on a viewer side and asecond polarizing plate provided on a back surface side; and a surfacelight source device for illuminating the liquid crystal display panelfrom the back surface side, wherein the surface light source devicecomprises: a light source unit; a light guide plate for causing lightfrom the light source unit to enter from a light incident surfaceopposed to the light source unit, and for outputting first directivitylight from a light output surface opposed to the liquid crystal displaypanel, the first directivity light being polarized light havingdirectivity of maximum intensity in a first direction, which forms apredetermined angle from a normal direction of the light output surfacein a plane approximately parallel to a light guiding direction of light,and having a high ratio of a polarized light component oscillating inthe plane; and a prism sheet arranged on a liquid crystal display panelside with respect to the light guide plate, the prism sheet comprising aprism portion in which a plurality of columnar unit prisms protruding ona light guide plate side are arrayed, the prism sheet being configuredto convert the first directivity light into second directivity lightdirected in a second direction within a predetermined angle from thenormal direction of the light output surface of the light guide platewhile substantially maintaining a polarization state of the firstdirectivity light, wherein a ridge line direction of the columnar unitprisms is approximately parallel to the light incident surface of thelight guide plate, wherein a transmission axis of the second polarizingplate is approximately parallel to a polarization direction of thesecond directivity light and the light guiding direction of the light inthe light guide plate, and wherein the transmission axis of the secondpolarizing plate is approximately perpendicular to a transmission axisof the first polarizing plate.
 2. A liquid crystal display apparatusaccording to claim 1, wherein the prism sheet comprises a base portionon the liquid crystal display panel side with respect to the prismportion, the base portion supporting the prism portion, andsubstantially having optical isotropy.
 3. A liquid crystal displayapparatus according to claim 1, wherein the first directivity lightcontains, by 52% or more, the polarized light component oscillating inthe plane approximately parallel to the light guiding direction of thelight.
 4. A liquid crystal display apparatus according to claim 1,wherein the light guide plate contains a light scattering material,wherein the light guide plate comprises a plurality of columnar backsurface-side unit optical elements protruding on a back surface side ofthe light guide plate, the plurality of columnar back surface-side unitoptical elements being arrayed from the light incident surface side ofthe light guide plate to a side surface on an opposite side, and whereinan array direction of the columnar unit prisms is approximately parallelto an array direction of the plurality of columnar back surface-sideunit optical elements.
 5. A liquid crystal display apparatus accordingclaim 1, further comprising a polarized light selective reflection sheetbetween the surface light source device and the first polarizing plate.